What you saw, @Chrysanthis, was someone whose layers of civilisation had peeled back. The raw person. Without shame, without restrictions on behaviour.
@StopStartStop Yes, very much this and it's no joke. The problem OP is that it's a crafted image and they hide it and can fool everyone around them, especially their partner in the beginning but for the partner that doesn't last. Sympathy and self doubt is one of many tools, it's emotional and psychological.
For people like this there is no empathy, remorse or guilt and they are justified in everything they do. They also do not like criticism, blame or being at fault and do not take kindly to opposing opinions. They can hide themselves but you'll find they say and do things others wouldn't and there is no feelings of remorse about doing so... it's like totally normal to do those things.
Dissociation or zoning out over a traumatic topic then being done and moving on right afterwards, I'm sorry but I find it hard to believe he could just carry on like normal after it. I can't just go oop all fine after experiencing that myself.
I never in my wildest dreams ever though that mine would physically hurt me but it happened often, even tried to suffocate me in my sleep a few times, then laughed like it was nothing, just something funny that happened one night. Those without empathy are capable of anything and when they go off anything could happen.
As others have said if this is the case be very wary of discussion because the wrong topic, word, tone or way you look at them is enough to unveil it all and you don't want to be around when that happens.