desensitizes them to violence. Kids are not able to distinguish between reality and fantasy, they may not understand that shooting someone will hurt or kill them, and they dont understand death and it's finality. may make them more violent acting out what they see.
I guess people can change but I wouldnt count on it, that is, I wouldnt bet my emotions on someone who has cheated on me before. Besides, once that trust and loyalty is broken, it's really over.