+1Playing is what kids need to do for healthy development. It is the parents job to keep tabs on their kids psychologically to avoid becoming the adam lanza type with a secretive lust for killing. My boys shoot each other all the time and switch off on whos the good guy and whos the bad guy. My younger son gets upset when his older brother and friends make him play bad guy alot. Thats a good psychological sign to note.
When my older son was heavily into Star Wars, his favorite character for the longest time was Anakin. He knew that Anakin becomes Darth Vader. Several times we talked about what the whole series was really about, good vs evil. What is gained by being good or evil and how your choices affect everyone else. It was during one of these conversations at 6 yrs old when he told me, "Dad, I know none of its real, I know the difference between fake and real, I just like how Anakin dresses...". A huge sigh of relief let out as I knew we were on the right track.
Parents just need to smartly be involved and actually give a **** about how their children are constantly developing.