Oh, I want to read books more, but it is way more dry, as people say, so I am holding.

Sure and thats actually good point, but in both cases, emotions won over him, so he lost control for short amount of time. And cases were also different, bacause Vader was already on the dark side and they were fighting. So, I think, from here comes his doubts.
Could not find subtitles, but as I heard, he said, that he sensed dark side (Here he tried to kill Kylo - because it was so strong, that he thought Snoke already turned him evil), but then that Darkness faded (Here he was ashamed).