Here is a thing I don't quite understand when it comes to him and that would be his suicide tendencies.

In Before Crisis, instead of Cloud "finishing him off", Sephiroth cut Jenova's head and jumped into the Lifestream with it. It is clear what's cannon and what's not but still. Furthermore few of his own clones in the main game killed themselves as well.

But by far the most jarring case was in Dissidia Duodecim where Ultimecia and Emperor Matheus tried to recruit him but Sephiroth refused (for quite noble reasons). After he had a fight with Matheus, Sephiroth then took his own life, on surprise for every other warrior. Even after Garland brought him back, Sephiroth was weirdly pissed about it.

(Also yes, I know he uses his left hand for sword. I did this on purpose.)