It's not just the characters though- it's the entire plot;
Why would the Engineers leave directions (supposedly)
to a system that they used as a weapons testing facility?
The melodrama between Peter Weyland, and his awful monster mash face
and Meredith Vickers is horribly written.
The Pathogen's space magic goo that can do whatever the plot requires.
Most of the dialogue is also... bad.
Like, Star Wars prequels- "people don't talk like that" bad.
Brett let the cat go, and at that point they were chasing something smaller than a Cat-
they had no way of knowing it could've grown to such ridiculous,
taller than a man proportions- in that time.