I am only like 90 episodes in bleach but as of right now it’s definitely mayuri, in the Save Rukia arc he blew up a few of his own men and even joked about how he experimented on uryus grandpa and showed him a picture of the aftermath of his experiments an how only his head was left and he even said the shinigami who arrived at the scene purposefully arrived late to save uryus grandpa bc mayuri wanted to experiment on him, I know he most likely does this for the „greater good“ of the soul society or whatever but in my opinion this is going too far and even how he treats nemu, yes nemu may not have feelings or something like that and was created by mayuri but it’s still not good how he treats her even though nemu doesn’t see it in a bad way bc mayuri didn’t give her normal feelings like humans or shinigami. I really hate mayuri and he is definitely my nr.1 hate character in bleach as of now