Darth Vader's psychology is, at least to me, actually interestingly complicated. Simplifying it: on the one hand, he's strangely loyal; on the other, he's filled with guilt and anger, and the Emperor let's him indulge in that. Also, he thinks he's on a path with no off-ramp, and then suddenly his son shows up and goes "No, there's good in you, I've felt it." Vader starts realizing he can build his own off-ramp, or at least jump off the path.
I'm not entirely convinced he was a good person in general even before he fell
You see, that's another bit that's so interesting to me. How much of Vader is Anakin, and how much is Sidious' manipulations?