In yu yu hakusho why did togaro kill genkai?

Answer 1

Togoro killed Genkai because, when they were younger, they were in love. They, along with his brother Elder Togoro, entered the Dark Tournament and eventually won it. As his wish, Togoro asked to be turned into the "highest class of demon" in order to punish himself for what he thought was his fault in letting his students die.

Genkai didn't agree with his decision or his self-blame, and instead asked to be left alone by the tournament committee. This angered Togoro, because he couldn't understand why she would allow herself to die when given the chance to live forever. Togoro was never able to understand why Genkai didn't choose to become a demon, and developed a lot of anger towards her.

When Togoro killed Genkai he looked at it as her own fault, since she could have chosen to be like him. He never realized that the reason she left him was to try and save him from his own guilt and self-loathing, although Yusuke is able to see it almost immediately.