He simply wanted to be left alone.
Sure, maybe he overreacted when anyone got too close, but it was only because the lack of solitude deeply angered him.
Maybe he should take some anger management classes, but then he'd have to get close to people and then he'd probably end up smashing someone to death.
It sort of helped that if a small animal got by him and messed with the knot, he'd be down.
Down but not out.
It was temporary, of course.
The point of this is that the Whomping Willow should just be left alone.
But if you decided to come near, he'd be more than happy to smash your brains in.