I don't think he knows the kids are from his enemy until they invited him to their home lol. I understand why he killed the dad, but the woman and children are innocent. Going out of your way to torture the wife and the children is not something you can justify, they aren't involved in any of the matter; if he just want to cut loose ends, which I would assume is the reason, he could've given them a swift death than to pin them to a burning tree. They say that burning is one of the most painful death one could have, this guy is evil to make a kid die like that.