Amnesia Doesn't Scare Me...

[Bridge]

Shooting people with a firearm, while traumatizing to a degree, isn't as traumatizing as axe-murdering people. Depicted violence also depends on how realistic it is. TF2 is more cartoonish than Condemned, but if you don't feel that something is wrong when killing in close combat in TF2, you should be concerned, because that means that you've grown desensitized to it, and that much more able to kill somebody in real life. In Condemned, the violence is so realistic, that by the time that you've grown completely used to the violence depicted there, you have the mindset of a murderer. If you can joke freely about killing people in Condemned, you can joke whilst killing people in real life.
NPCs are irrelevant only when they are percieved and thought of as irrelevant. While playing, your brain is being programmed as if the situation was real. It is learning a real-life behavior, and it's especially good at it when stress is involved. This happens even if you would be thinking that it's only a game while playing, which you are not.

Sorry but the view you are taking is not realistic. It's called role playing dude. I am completely able to step into different roles for the purposes of entertainment or education for a certain period of time each day. Being in a different mindset for a few of hours (even many hours) a day is not going to have any lasting impact (and it's certainly not been scientifically proven that it will). It takes a lot more than that (say, an existing mental disorder) to lead to complete derealization like that. Is it disturbing to kill someone with an axe? Yes it is, and even raving lunatics have an emotional response to that, but the human mind is extremely powerful and making it not disturbing is what makes role playing possible in the first place. So I might place myself in a chaotic evil role every now and then; doesn't mean I don't stay chaotic neutral with good tendencies when the game in question comes to a conclusion.