Murderers and rapists may not have the free will to do otherwise, but we should still put them in prison, just like we keep a lion in a cage. The lion doesn't know why, he's just acting by his instincts. A compatibilist approach could have moral responsibility and have deterministic ideas about murderers. I don't subscribe to compatibilism, but it doesn't mean you must believe in free will because you don't think determinism cuts it.
The universe operates mostly by determinism, a chain of cause and effect that we can't break into with an uncaused cause, so there can be no free will. The parts of the universe that are random or indeterminate are, by definition, not within our control either, so there is still no free will. The only "proof" of free will that proponents can offer is that it is "obvious," forgetting how easy it is for us to be fooled.