Sigh. It feels like we've had this discussion before a lot of times. And still people are like "the games don't put a gun in your hand, they would have snapped anyhow".
No. Games don't make people kill other people. But a person who has spent their entire childhood watching depictions of murder in various forms a few million times may become desensitised to violence. Of course, this is very hard to prove, but it wouldn't be a surprise to any of us. The first time you see a dead person, you're likely to react with fear. But due to diminishing returns, that shock factor is lessened for each and every time you're subjected to it. Why couldn't this also apply to games? Maybe having spent your childhood killing virtual people has devalued your appreciation of other human lives? Would that effect really be impossible? I don't think so.
Put someone with low mental barriers against using violence in a highly stressful situation and something bad is likely to happen.