In the case of my ex he is on a path of self destruction to prove to the world he "doesn't care" but I think it's attention seeking and mental instability.
I think people too often are ruled by their intense emotions, and that brings out the negativity.
Depends on what their intention is. Sometimes you can't build if you don't first destroy.
I used to wonder the same thing, until I discovered that their reasons usually make very little sense.