So he beats the shit out of her flat and comes round to yours, cut and bleeding?
Did the DC see him in this state?
OK, calm. Calm.
Tell him that in light of his behaviour, you don't want him coming to see you or the Dc unless arranged in advance.
Remind him that it was HIS choice to leave, HIS choice of partner, job, etc,
Remind him it was HIS choice to smash up her flat, and that your only mutual interest is the DC. Any subject outside of this will not be a part of any conversation. Tell him that you are putting it simply for the avoidance of confusion.
You do not care, nor want to hear about his other life, he made it so, he deals with it. If he refuses to see the DC, they will eventually get over it and they will see HIM for the deadbeat dad that this action would make him. Tell him it's HIS choice what kind of father he will turn out to be.
Then that is it. Totally disengage, get a new mobile and only allow him to contact you regarding time with the DCs.
Cancel the Christmas lunch with him and find something else to do. Could you spend it with your ILs?
Cut contact with him unless you are convinced he is safe to speak to.