Decency implies that one has a sense of what is good and bad. A moral compass. But for an abuser, what matters most to them is themselves. As long as they profit or benefit from something, they are willing to do anything or let anything happen.
Everybody is responsible form themselves. Everybody is responsible for fighting ther own demons, go to therapy for their psychological issues, go to rehab or AA meetings to deal with addictions. Abusers refuse to accept this responsibility.