Its not normal. even for fun.
I was pretty much the same at that age. Love violence. I've always been a sadist....
My ex's son suffered trauma from birth to four. Diagnosed with PTSD, but parents did not get help and made it worse. The "artwork" from this boy was horrifying. He planned out gruesome attacks and sexual stalkings and did not know what was real. No understanding of consequences, either. I fully expect him to be in the national news, having committed one of the country's worst atrocities. It seems inevitable yet was so preventable.
Definitely might want to get some clarification as to why...
It depends, has he been playing Violent Video games,Watching Violent Movies. He could just be Drawing what he has Seen.
Maybe he's being bullied by his friends or whatever.
He's letting out his emotions and that's how he is doing it!
Better to draw it than do it