Ok it's still bugging me

Psych 101 wouldn't even make reference to anything in this movie. No undergrad psych class would. You are also incorrect about people not being able to kill people if there life is on the line. Generally the sympathetic nervous system will take over and if the person does not run away, they will kill to defend themselves. The sympathetic nervous system takes control of the body and the person loses control over their reactions. That is psych 101.

For a psych student you don't seem very open minded... I think you need to realize movies aren't reality. They create a premise and exsist within that premise, outside of that it doesn't serve any function besides being entertaining. You can't analyze a horror movie within your definition of reality (what is realistic behaviour) and say "people wouldn't behave like that" cause well frankly you don't know how they would behave.

First off, the flaw, as some might call it, is the most predictable part of any horror movie. About 99% of the horror films I have seen, have at one point had a scene where the main character(s) is given the perfect opportunity to kill the bad guy, but they rarely do. Is this simply because of their inability to kill? I don't think so. I think it's more a movie concept, wanting to keep the story going perhaps. For instance if she killed the bad guy in this movie during that scene it would have ended after an hour, instead they kept going.

Secondly, I don't wanna break out in a psych showdown here, and you might know more about it then I do, but the theoretical aspects of psychology are based on ideal human behaviour. Humans want to believe that, even in the most grusome scenarios humans would still treat each other with dignity and respect, but as I'm sure you know (if you are a psych student) that's not the case. Take the Stanford prison experiments. Within a very short period of time people went from being normal, caring human beings to creating a class system were the prison guards intentionally and willingly mistreated the prisoners to such an extent nobody were able to predict.

I think you're blinded by the theoretical aspects of psychology. There hasn't exactly been a realistic study on what people would actually do if they were placed in a situation like this (for obvious ethical reasons...) People who have experienced horrifying things might have said they weren't able to kill, but that doesn't mean it's the same for everyone. The one thing psychology can't (won't) claim is that everyone is identical. People react differently, they deal with situations differently, so it's impossible, yeah I said impossible to predict what someone might do in a case like this. Regardless of what a text book might say, the truth is they don't have a clue! The closest prediction of plausible human behaviour in terms of films would be The Hills Have Eyes, the closing scene of the original really showing what humans MIGHT do if pushed too far. That's really all you can do, it's predict what they might do but you need to do it within the premise the film sets for its action. By premise I mean the scenario/plot it has presented.

In regards to this film in particular I agree with people saying it completely threw off the flow of the movie. In that moment you want them to kill the bad guy, you expect them to kill the bad guy. She already shot him proving she is capable of hurting someone, so it wouldn't have been the most improbable thing in the world to have her kill him. She did in fact try to shot him again but the gun jammed/ran out of bullets so she resorted to playfully hitting him on the back and running away...

Lastly, this isn't intended as a attack on you personally, I'm just trying to have a good discussion about the movie and the claims you made. Hope you don't take it personally.

Also as a last, LAST point in this argument. You said earlier people are unlikely to kill (did you mean hurt as well?) even if they feel threatened. I don't know if this counts but studies have found that in cases were people have percieved themselves as threatened, your entire body actually prepares for a fight. Muscles clench, heart beats faster, breathing is faster, adrenaline starts pumping throughout the body. The body is prepared to deal with a fight. Obviously it's quite a stretch to go from fighting to killing someone but in the heat of the moment it's certainly plausible that you might kill someone if you feel threatened. Maybe I'm wrong but then again, maybe you're wrong...? Who knows...