I'm afraid that's just boys!
My first son wouldn't have been into anything like that at 3. However, as soon as he went to nursery, and then on to school - all he ever plays (at school) is fighting/army/superheroes etc etc. Consequently he comes home and plays with his younger brother (who has just turned 3) and it's all about 'I'll kill you' etc etc. So my 3 yr old can be heard saying similar at toddler groups, despite my best efforts to stop him!
At first I was utterly horrified, and would explain that kill isn't a nice word etc etc. However, my H is much like yours and has pointed out that it's all just part of it. They're also much more into playing rough and tumble now, and pretend punch ups. Not my idea of fun, but I'm adjusting to it (especially as I have 3 boys, so when L is bigger no doubt he'll be doing it too!). They understand that it's playing and pretend, but I can't make them not want to play like that.
BTW - mine don't have guns/swords or anything like that, I don't let them in the house. So they make them out of lego, eat pizza into gun shapes 