So he was already there when you got to the camp?
Yeah it sucks when you want to do something and someone else is already there. In this case I think you're perfectly justified picking off mobs from his train here and there. But I don't think you can really complain about it, especially if you were the 2nd one there. Also, it doesn't sound like powerleveling is really relevant here, you just didn't like someone else killing things at a camp you wanted.