My character Kat was so badly affected that she suffered penalties to her attack rolls (FATE game, milked that for fate points <3).  She was a thief, and she'd fight, but she'd never killed anyone up close and personal (she'd possibly gotten the final shot with a gun, but I think in the heat of a battle she might not notice them go down and might not know they're dead, not just injured).

My character Raia was...not happy, but she has a very strong moral and ethical compass, and she saw that person as wicked.  She handled it well, though she had a few nightmares after.

Max has an interesting story.  He's actually a sworn pacifist, and it's magically enforced (side effect of the holy healing magic he's been granted.  He's a doctor).  However before he received that boon, he was possessed and attacked someone.  He didn't remember that, but found out after the fact.  Then he was stuck in a dimension that responds to fears, so he had to watch himself do it over and over....that whole series traumatized him quite a bit.  However, the first person he actually killed was a matter of a cancer patient who was in a great deal of pain and didn't have long to live anyhow.  She wanted him to, so he used anesthetic to send her on her way.  The two were good friends, and he was shaken and upset, but not significantly more than he would have been by her death in general.


I agree with everything [@Shoryu Magami] has said.  Another thing that might happen is nightmares, as I mentioned, or perhaps the image of that final blow replaying over and over in their head, either right away, or in similar situations.

There is, however, the other side of the coin.  Some people like killing, it gives them a rush.  For whatever reason, perhaps the character is excited.  A thrill of adrenaline, a surge of satisfaction...not a happy thought, but a possibility.  There's also some who wouldn't even care.  The other guy's dead, so what.  Or maybe because it's "them or me" it doesn't bother them.

Hatred of the cause that resulted in that situation ending in the kill is another idea.  It's always easier to blame someone else.  Perhaps a soldier hates a country or a government that sends people off to fight while sitting safe at home in peace.  Perhaps a homeless person hates the system that forces him to steal to survive, and how he wouldn't have been trying ot rob that store if he could just get a job.  Why does nobody understand how hard it is?  Why do they all think it's just a matter of trying harder?  In an extreme case, the killer may end up cynical and jaded, a pessimist, and even hate humanity as a whole.