Maybe the message is, "it's fun to kill people you sick fuck". Especially if you don't see them as 'people' or humans. Hard to convey something different in a video game. Even from the age of six years going, as proven, children can already differentiate between reality and fiction and know "this ain't real people". And if you don't have that very import aspect, killing is probably as easy as breathing (see several genocides because those cultures/peoples weren't seen as 'humans', Christians, believers, etc; the brutal shit humans did and still do to other humans? That's only possible with no attachment or care whatsoever for the opponent as a living being, or is only possible with great rage and anger).
Plus, there's probably something destructive well within our DNA, that make even small children enjoy destroying towers made of building bricks by other children and find it fun, especially when the other child starts crying. Behind our 'civilized' mask there's a primeval, antediluvian, brutal and violent truth that wants to come out and is probably never going to be erased but only impeded by education, laws and acquired social norms and conscience.