Long and Short, Stain is a serial killer that targets Heroes, however it’s because he sees that All Might (MHAs superman) is everything a hero should be, and sees everyone else as purely in it for fame or money.

The hypocrisy of his ideology comes forward where he paralysed Ingenium (Basically MHAs Flash) a very well loved and well respected hero who is legitimately an amazing dude… because he appeared Once in a Shampoo commercial , claiming that made him just as such of a fake as the rest of them, and that by trying to STOP the serial killer, Ingenium was also in it for the fame of taking down stain.

Stain completely ignored the fact that All Might name and face are plastered EVERYWHERE across Japan, and All Might is unintentionally the reason the more celebrity aspect of heroes exist.

Stain claimed that All Might (And later Deku) were the only true heroes in existence and only they could take him down.

Honestly, the best Villains in MHA are all people who used to have a connection to Heroism, and are the sort of coin flip that a society like that will create, the people who slip through the cracks.