I agree with him that the idea of 'let's teach them a lesson' does sound a lot like vengeance to me. Even if what they are doing is technically self defense, if they enter into it with a vengeful attitude then the technicalities don't matter. God is all about the heart.
So the internal intent of the action is important. It may not matter what it looks like; rather, at a religious level, it matters WHY a person is doing it.
Common ground. There it is.