Violence has always been a part of "play"--and we as a human race seem to have survived thus far without breeding a massive army of cold, unforgiving, killing machines who see murder as childplay.
I've had a lot of time to observe my 7-yo with her friends playing in our shared lawn (I can see them from my home office) and I find it fascinating that the most "violent" weapon-like toys they have are water guns and there is no sign of good x bad roles in their games or even the notion that the main goal is to shoot at each other.
My own childhood was much more violent, with cap guns and always present roles of deadly antagonism.
