Ok, I'm in the middle of watching Star Trek (the new movie, ILLEGAL RIP OMGZ) and I've reached the scene on the planet with the person and the ice, if you've seen it you know the one, and the part where the bigger monster comes out and kills the littler but no less fangy monster that was previously pursuing... the person. Trying to be non-specific here, but my point can be made without specifics;

Who the hell ever invented that trope? The one where you're getting chased by a big monster and oh noez, mortal peril! Until suddenly! A bigger and even more terrifying monster kills the first one! But you're not safe yet! Because then it starts CHASING YOU!!!

This doesn't make sense from any standpoint. You're prey, the proponents of this shortcut to tension say! Small running thing equals food! Except the lie is built right into that argument because oh yes! The bigger creature just killed a much larger and eminently more toothsome morsel than the fleeing party could ever be!

Oh, then they're protecting their territory then! Except you are so relatively small, always, in comparison to big bad monster, that for that particular argument to be taken seriously it would have to be established that the particular creature is just SO unhinged and territorial and aggressive that it attacks anything that moves. Now, this does happen (rarely) in nature, but frankly its so lazy to just assume that big thing will be at all interested in Hero A's tiny, un-threatening ass  that I am not at all inclined to give those particular writers the benefit of the doubt.

No one is safe from this sad little cliche; dinosaurs, crocodiles, giant unnamed sea creatures, fangy reptilian land beasts of indeterminate origin, overgrown insectile nightmares... 

For god's sake, writers, let the poor creatures eat in peace!