I think because he got no choice, to be able to learn how to fight he must learn from his enemy. I also think this is kinda make sense because by joining pirates he also able to learn their weaknesses?
Well, as you can see, they are treating him fairly well. He gets food, protection and he gets to join their battles and learns how to fight. Why would he abandon that for being alone in the wilderness, perhaps steal to survive, and maybe end up dead to either the wilderness or the Englishmen?
when he was killing before that old women he looked like a demon spawn
maybe she must've thought that she made a mistake to save him
I'm just imagining what happened to that old lady and her grand-daughter probably raped and killed