There's really no excusing him holding a pregnant woman at knifepoint, but James had no idea what he was getting himself into when he was lead to the pillhouse, and what he saw undoubtedly broke his mind. He was, however, not in error when he killed Powell, although that didn't last long.
Vincent, for all his aggression towards the entire cast (including Simon at one point), has a sympathetic backstory, so it's understandable why he is the way he is.
Lost in Medias Res: To unfun degree and the movie clearly suffers from the 90 minutes of running time, while having material for full two hours. While the film opens in the middle of the plot, almost nothing is explained by the end of it (despite countless lenghty expositions spread all over the movie) and numerous things are only hinted. By the third act the sheer weight of elements left hanging collapses the plot, leading to incoherent mess regarding the cult activity. And then there are all the minor, personal details about characters that never are elaborated or used, creating another batch of unanswered questions.
Nausea Fuel: The creature effects are ... rather unpleasant to look at. Some examples include Mutant!Beverly being axed and pus oozing out, the mutated Beverly drilling her tentacles into Mitchell's eyes and mouth ...
Special Effect Failure: While almost all effects are done using practical means and are really good, the greenscreen showing empty desert behind Daniel and Allison in the final scene is almost painfully obvious.