Like Psylis said, if you kill people in a video game it's not considered a sin (or atleast many would not considere it to be). And I'm pretty much just repeating what others have said-
If you feel bad doing it in a dream, then in my opinion- don't do it. Even when I'm not in control of my dreams and my dream self does something bad to a person (depending on the person ), I'll a bit feel bad upon waking up. Dreams gives up deeper emotions because they are "real" when we experience them. Video games are not real and would normaly not gives us the same feelings.