or is it just getting rid of steam and anger?
I think it's just a knee-jerk reaction TO the anger.
Also, at the time they are saying these kinds of things, yes, I believe they believe it. And 5 minutes later, once their emotions have changed, they no longer believe it and will possibly deny ever saying it. Or claim you said it. Whatever can be done to distance themselves from the shame of ever saying or doing anything wrong.
It's far easier to criticize you, to pick at you, to make all problems in your shared lives about you, than it ever would be to look inward.
I have a term in my head - it's by no means a "real" term, but I describe it as being in the BPD Bubble. Once you are in the bubble, that's when you start to see the BBPD behavior that brings us to places like bpdfamily to learn more. Like this - H is super critical of his own clothing. When he is feeling REALLY bad about his own appearance, he tries to micromanage what I choose to wear, to avoid talking about how he feels ugly. He makes it about ME, and can often say things that are not very nice, but they are more often what he is thinking about himself.