A lot of people are complaining that Unohana's ruthless depiction was ruined, but that's only because because of them thinking that the gentle, caring Unohana wasn't real, nothing but a mask, yet that is also a part of her, she is regretful, she is caring, she has changed.
At the same time, she loves or loved battle in a way similar to Kenpachi, so seeing that her own weakness forced a warrior of such potential to restrain himself, to limit his power instead of rising to his full potential, to basically stop being himself was a great hit, that's her sin.
Kenpachi for his part basically lived to battle, yet never had a challenge, he was always too strong.
When Unohana appeared, he finally thought that he could fight without restraint, to finally let loose. When he won, he realised that it ended the same way as every one of his past fights, but the fun he had during this one was too great, he didn't want it to end, he wanted to go on, if he was too strong for anybody to defeat him, then he would become weak, so that he could enjoy the feeling of being able to fight to the limit forever.
It's more of a mental trauma than a conscious action, he doesn't know he's doing it, but his body is fighting with less and less restraint every time he is about to die, he's basically readjusting to the level of the fight.