Alien invasions

Some alien films straddle the line between horror and scifi. While not all great, they all usually have a unifying characteristic: Aliens don't like humans, and are gonna wipe us out in a horrible way. One movie that fits this mold that I saw for the first time recently was 'Alien'. Between the chestbursters and the facehuggers, I was always too terrified to watch that movie. But the motivation of the alien beings in that film makes sense. Same thing with 'Maximum Overdrive'. Stephen King directed this 1986 film about a group of survivors holed up in a truck stop. All the worlds machines from the smallest (a tape player) to the largest (a suspension bridge) have minds of their own and are killing people. While not confirmed, it is the theory of Billy (played by Emilio Estevez) that an alien race is using our technology against us to exterminate us, because they want our planet for their own. This makes total sense to me. They kinda did that in 'Independence Day' too. However, there are some alien invasions that make no sense whatsoever.
Take James Gunn's 2006 gore-fest 'Slither'. I rented this movie purely because Nathan "Captain Tightpants" Fillion and the gorgeous Elizabeth Banks were in it. It's honestly not a bad movie. It's really funny, and you're into gore it's totally your bag. The thing I have a problem with is the alien's cycle of destruction. Basically, it goes from planet to planet. Some it impregnates, some it infects, and the rest it eats, until the planet is dead. Than it moves on. Why? We never find out why. It just does. And it's been doing this for over a million years. This creatures sole purpose is to destroy planets through gluttony and osmosis. Seems like kind of a dick move.
The other alien invasion story line that makes no sense is 'Signs'. Granted this is an M. Night film, and should be taken with a bag of salt. But if I was gonna try and conquer an alien planet, I'd do a little research on it first. The aliens in this film are basically allergic to water. The human body is made of 55-78% water, depending on your size. Water covers over 70% of Earth's surface. It's in our atmosphere and it occasionally falls from the sky in various forms. How the hell did these aliens miss something like that? Our planet is basically worthless to them! It would be like if I tried to take over a planet made of fire. Even if I succeeded, I wouldn't be able to do anything with it. Way to think things through, aliens?!