While it is true that abusers can beg and cry for forgiveness, this may not be the correct story. We really don’t have enough information to really guess what the correct story is.
Why was he mad at you? How mad was he? If you had kissed his best friend or been unfaithful, I can see him posting such an ad out of anger.
Did he get any responses? Did he follow up on any of them? He may have only posted the ad to get back at you, but had no intention of having nsa sex.
What behaviors do you mean when you say “lash out so cruelly in one form or another?” Is it verbal lashing out? Is it about whatever it was you did to make him angry? Or is it generally casting aspersions on your character for no particular reason other than he’s angry? Is he angry in general? Does he drink too much, or use other drugs?
What’s really going on? Maybe he’s guilty about something he did. Maybe he’s truly very angry with you. Maybe he’s abusive (you would know this better than any of us).
Whatever it is, it sounds like both of you have a lot to learn about relationships. You’re not going to learn it on your own, as things stand now. If you do want to stay in the relationship, you’d do well to enter couples counseling. Otherwise, you will probably do this kind of thing over and over.