Most people would say that Kurotsuchi Mayuri is evil, and there's no denying that he's done some pretty terrible things. He's experimented on countless humans and Shinigami, without any regard for their safety or well-being, and he seems to take pleasure in causing pain. However, it's worth noting that Mayuri isn't purely motivated by malice - he genuinely believes that his experiments are necessary, in order to learn more about the world and find a way to defeat the Hollows. In his own twisted way, Mayuri is trying to do what he thinks is right, even if his methods are cruel. So while you could certainly argue that Kurotsuchi Mayuri is evil, it's not quite as simple as that.