Maybe in the game they develop a sense of control which carries on in their dreams. Maybe it helps detach them from something personal as killing in video games tends to be impersonal, which would mean that impersonal violence carries on in their dreams so that instead of seeing the eyes of someone you killed or saw die, it becomes a blur and they see many blurry enemies or allies instead of that one defined one that haunts them. Or maybe it helps them see things clearly and develop battlefield awareness playing video games that helps them see what is going on in the dreams and gives them security that they can find safety.
If aggressive video games help soldiers with ptsd, should a woman who was raped play a video game where there is a lot of rape in order to cope with it better in her dreams? So there is some irony there. But hell, maybe so.