My dad is a nonviolent man who played war, cowboys and indians, play hunting, and cops and robbers in the 1950s.
My husband is the same.
Play shooting to small kids is not the same as wanting to actually shoot a person in real life (Edited to add: please see my clarification/qualification below in post #32) . In kids games, they shoot you and you die and then you get back up and play again. It's all pretend. Sure it's not a pretty pretend, but it's working out the complex dance of aggression and right and wrong and where they fit in the world around them. My general feeling is that everyone clears the rules of the game with everyone and everyone participating is willing, then let it be. What is NOT OK is involving people who do not want to be involved, or pretendign to involve random strangers not in the group of people playing. I used to have an "only shoot at pretend monsters/scary pretend creatures" rule, but found that in some situations my son and his friend wanted to be on the same team doing that, but sometimes they just wanted to shoot each other and pretend to die and then get back up and let the other have a turn.
One time when I got a little upset at one of the things they were doing, one of the first times they were doing it, my son said, "sheesh mom, it's not real, we're not really shooting each other - it's just pretend like the other stuff we do." We ahve had numerous conversations about pretend vs reality, and books and tv and how what happens in those things often couldn't happen in real life, and about actual guns and what they are for and what they do. And this kind of pretend play where one is a storm trooper and the other is a rebel has NOTHING to do with that.
I dunno. I still have mixed feelings about it - However, I am NOT worried that my 6 yo son playing games like this at this age, the way he plays them, means that he's going to be indifferent to people being killed as a teen or adult. Kids have been playing at violence since there were kids in social groups to begin with. So long as there's accompanying, non guilting/shaming conversations about reality vs pretend, and about real safety against actual violence against living people and animals, I don't see a problem with pretend play of this sort so long as all players are in agreement...and I further see how preventing this play entirely could backfire in a forbidden fruit kind of way. And FWIW, I see little different between being dead from a sword and being dead from a gun. Dead is dead. One requires more skill and proximity than the other, but the end result is the same.