Great scene from a great movie

So, the character says he will never be on the wrong side again. But he gets tricked later on, and this is what gets William Wallce captured. So, his father noted what he said in this scene, and found a way to trick him so that Wallace could be eliminated. What came later was another scene in which he confronts his father about the trick that got Wallace captured.

His father replies, "you let yourself be deceived". Indeed, that is how deception works. If you let yourself be deceived, then it will happen. Deception depends upon that happening.

Easy to promise to not do (or not do) something, but much harder to actually accomplish it.