This is a very good question that I have been asking myself for many years. These games are definitely not healthy and cannot be beneficial to the human psyche at all, especially in children.
I think that culture in general has led us to see it as "normal" for children to play with weapons, swords, bows and arrows and so on, I suppose because of the whole history of wars that we have inherited from many centuries ago.
I feel that we parents have a lot of responsibility in allowing children to play this kind of violent games, instead of giving them educational and recreational tools and means that are appropriate to their age.
We need as a society to overcome all these paradigms and learn to teach our children the path of peace and not that of war.