i mean from the perspective of saike, someone who self-sacrifices to the extreme to help people out, and definitely wants for the greater good, i can see why he'd perceive the exact, total opposite as evil.
but you've also got to remember that he's also behind the shenanigans of the other villains, is abusing to an excessive degree a mind control ability, and no matter how you swing it, him just "fooling around" is going to ruin lives, intentionally or otherwise, with the victims having no idea what the fuck is even going on. the end result isnt really as important as how he plans to get there, and how he plans to get there is pretty blatantly immoral.
and then, you know what they say: "evil is petty" and whats more petty than upending countless lives in what is ultimately a truly juvenile fantasy that benefits no one but himself.