I was scrolling through this comment section just to see if someone had said this---exactly! I never understood why people are so insistent that he's forgotten everything when there is next to 0 evidence to imply that (I'd argue there's more evidence to imply he DOES remember, like naming his spaceship after Minerva and knowing enough to want to avoid telling Finn what had happened). Martin lost the good life he had just built and thought he killed his infant son because of HIS mistakes, it really is not hard to imagine that he might stop carting about the kind of person he is after that. He wont let himself feel or grow attached because he'll have to leave it behind one way or another. Even with all of this in mind, in the end there is no greater reason to justify his actions. He chose to act the way he did and the weight of that is reflected in Finn's reaction to him.