You see, no place is safe from them ‘orrible monsters!

We all have the right to eat, some monsters exercise this right on humans. Most monsters eat us because we’re the most edible thing around, or we possess a certain element which cannot be found in other creatures, such as human brains, like the Living Dead or Frank Henenlotter’s "Brain Damage". (Although I’d claim that most of the victims in "Brain Damage" seemed to lack any in the first place)

In rare cases, the monsters see potential which is beyond "let’s just eat ‘em", like in "Bad Taste" where humans are the doomed to be intergalactic fast food.

Dracula is the archetype of the drinker. Vampires portray the need for something humans can offer. They need it to exist. It is not a feeling of animosity towards humans that makes them drink our blood. As a result, humans are not supposed to die from blood loss, they turn into the undead themselves. This is a more positive fate than being eaten, wouldn’t you agree?

Some monsters, such as the Predator reverse our logic of things. As in one episode of "The Twilight Zone" where human astronauts find themselves locked inside a house-like cage in order to be displayed for the amusement of the Martians, the Predator is a superior being to most humans, and goes on a killing spree just for fun.

Alien is like a sophisticated spider. It only seems to kill humans. After a while we discover it actually uses them in order for its fetus phase to incubate. At the end of this process the human host dies.

In a twist on that notion the Body Snatchers use the human body because that is the only way they can reproduce. Of course, bodily possession is not the end, just the means. But the price humans pay is their own control of their bodies. A death within, if you wish.

Another form of possession is the straight soul possession. A favorite for the Devil, as he proves in "The Exorcist". Sometimes there’s no need even for taking control over an innocent person, an evil child will do, such as Damien.

Sometimes all that aliens want is to inhabit our planet. Some prefer the violent way, as witnessed in the likes of "War of the Worlds" and "Independence Day". Sometimes they just want to live peacefully beside us, like the creatures of "Cocoon", or "Alien Nation". Others are just looking for world domination, like the children from "Village of the Damned". Whether peaceful or not, aliens pose a basic threat to humans, invoking a natural sense of alarm. But sometimes confrontation is due to fear and hatred of the other. Classic case is ET (yep, ET is also a monster).

A personal interest for monsters can be object possession. In John Carpenter’s "The Fog" the monsters stalk the town until they find what they’re looking for. In "Gorgo" all the havoc starts when the worried dinosaur mother goes to save her child. In these cases humans suffer, because they were not meant to live side by side with these monsters.

Ghosts usually utilize violence to drive people away from their domains. "The Amytiville Horror" and countless other horror flicks are based on scare. People don’t necessarily die because of that. If they do, it's usually their own fault.

Revenge is, to my knowledge, the most popular reason for monsters to behave the way they do. Frankenstein’s monster vows revenge on humanity. The 50ft Woman avenges her philandering husband. Some say the Birds take their revenge on humanity because of its crimes against nature. Freddy Krueger avenges his death. I believe revenge is so popular because it offers a possibility that the monster may have a point of its own, making it more difficult to automatically identify with humans.

The psychotic monster sometimes looks for love. The "It’s Alive" baby is looking for his loving mama. Those who can’t offer the hideous baby love, must die.

So we learn, that various motives make monsters tick. But then again, we can see that not all monsters are the ones who started it. Why do monsters suddenly appear?

The way I see it there are three reasons for monsters to appear:

Pure Coincidence

This is the least common reason for a monster. It is highly unlikely that monsters would just pop out of nowhere with no connection to anything. But you know, accidents will happen. One good example is "Invasion of the Body Snatchers". The spores while flying in space eventually reached Earth causing what they did. It does increase the tension by letting us know that no matter how careful we are, there’s always a chance... Also in that category, there’s the 50ft Woman, the Fly and the super intelligent Ants from "Phase IV". In cases like "The Birds", we can only speculate what the reasons were, because the Hitchcock doesn't give any clues. I find this case to be perhaps the scariest, because there's no moral justification for the horrbile events. We can't prevent from things to happen.

Personal Interest

A lot of monsters do what they do to us, because they want to. They may want all kinds of things. All of this is covered in the previous category of "What is it they want from us?"

Monsters are a Price to Pay

More monsters than you’d imagine appear because of our faults. Sometimes it’s directly our fault, like the Golem or Frankenstein’s Monster, which are monsters created by men. Their behavior is another issue, but they existed, because they were created that way by men.
Note that some monsters are created for the benefit of mankind, like Frankenstein's Monster (pure scientific research), or HAL, while others, like Mechagodzilla, for example, are weapons. Nonetheless, they all eventually turn on their creators.

There’s also the indirect reason, like the Giant ants from "Them". It’s messing with science and nature that created a mutation which is a threat to mankind. A kind of "backfire" of nature against us. You’ll find a lot of those around:

Messing with Science: Tarantula, Them, It’s Alive, Night of the Living Dead, Fire Starter

Messing with Nature: King Kong, Gorgo, Lost Weekend

Sticking our Nose where It doesn’t Belong: Forbidden Planet, Alien, Night of the Demon, The Thing (Original Howard Hawk’s Version)

Being Plain Bad: The Brood

This is the place where we really ought to learn a lesson. Knowledge comes for a price. We have to be curious about everything surrounding us. But we must be aware of the potential danger of hasty decisions.

Intelligence

Much of the conflict is determined by the monster's intelligence.
If the monster is intelligent, the hero must be smarter than it. One would think it to be harder than a non-intelligent one.
Wrong. The problem with a so-called "stupid" monster lies in its unpredictability. It's possible to confront an intelligent monster face-to-face, if one is sure what its reaction would be. Not so with the stupid monster.

A few telling examples:

The Predator is the intelligent monster. It has an intelligence akin to humans. That's why Schwartzenneger (in I) and Danny Glover (in II) can confront it. Actually, in both films one can only beat the Predator in its own field.

In the first two films of the Alien trilogy, the creature seemed to be operating on instinct alone. The 3rd showed the Alien had some awareness & utilized intelligence to a certain extent, at least not to kill Sigourney Weaver, who carried its fetus. The 4th stepped even further, as one Alien kills the scientist who tried to tame it, using the same taming methods.

Frankenstein's Monster is, in my opinion, the intelligent monster, at least as portrayed in the book, and if I remember well, in Kenneth Branagh's version. Unlike the so-called super intelligent monsters from outer space, this is an evolving creature, who, in very short period turns into a thinking person. As time progresses, the monster developes conscience, philosophical awareness, finally outdoes Victor Frankenstein himself. Eventually, the monster cannot tolerate the conflict between its evolution, and the social reality as an outcast. The monster is the only creature which can destroy itself. That makes for an awesome monster.

Most animal monsters are fairly stupid. But in "Birds" they have a blind collective power. It would be safe to assume the most degenerate of 'em all is the Blob. It's automatically expanding, swallowing everything in its way.

Countermeasures

Well, that’s a tough one, since we have to understand the monster’s weak spot. I mean, if we could just incinerate it, fine. But usually, that wouldn’t be much of a monster, would it?

Some monsters seem to have no weak spot at all. Take Godzilla, f’rinstance. The entire Japanese army (wow!) couldn’t kill him in the first movie, and he came again and again (Well, he actually got friendly along the way). Other monsters are vulnerable as individuals, but not as a collective mass. For example, many zombies were killed during "Night of the Living Dead", but they had the upper hand eventually. The problem with other monsters seem to lie with their alien physiology. We simply don’t know where weak spot is, and they’re not gonna just sit around waiting for us to find out.

Let’s examine various methods of disposing with monsters (warning: spoilers ahead!):

Ammunition

Usually, most monsters will be immune to gunshots, otherwise the movie will end 5 minutes after it started. But there are exceptions, like a bullet in the head for the zombie. Human monsters are also vulnerable to normal ammunition, but where the hell did I put my gun?

Fire

Can be a very useful weapon indeed. Quite a few monsters are vulnerable to fire. Others don’t pay any attention. The funny thing is that, unlike gunshots, if someone uses fire against a monster, it is bound to destroy it. When fire is useless, we know it, simply because it is the monster’s weapon in the first place (the dragon), or it casually passes through it.

The metaphysical weapon

I call it "metaphysical", since there’s absolutely no scientific reason why, for example, you’ll need a silver bullet to kill the Wolfman. Other metaphysical weapons are the famous wooden stake in the heart of the vampire, erasing the holy words from the head of the Golem, or special chants for evil spirits.

Scientific Warfare