From Romans 14:
14I know and am convinced in the Lord Jesus that nothing is unclean in itself; but to him who thinks anything to be unclean, to him it is unclean.
23But he who doubts is condemned if he (plays violent games), because his (gaming) is not from faith; and whatever is not from faith is sin.
(Basically, if you think it's wrong, and you do it anyway, then it's a sin.)
WWII games vary a lot in their violence/language content, so I wouldn't say that the whole genre is ok/not ok.
I also think that some people might be predisposed to violent thoughts and feelings; perhaps those people wouldn't have as much leeway in the games they were "allowed" to play. So I think a specific game could be okay for one person and wrong for another. My advice would be to pay attention to what convicts you personally.