I don’t think there is much of a thought process. Or at least his actions are emotionally consistent but not intellectual consistent.

Being broken up with is painful, either as a sense of loss or as ego pain.

In order to make both disappear he needs to get back with you, but if he only cares about making the ego pain disappear he just needs to convince himself he is better off without you, and the easy way to convince yourself of something is to convince others.

So both actions are just him responding to (what I assume is) ego pain, neither of them are necessarily informed by his true perception of who you are.

Edit: none of this excuses his actions btw, it’s meant as a way to understand why he is saying cruel things and to understand that the things he is saying have no basis in reality.

How did your boyfriend talk about you to third parties while you were together?