Mental illness has always been common and often been cool in one form or another.
Serial killers get fan mail, that’s true now and it was true 100 years ago.
I think a lot of people still grow out of that phase. Like wanting to be like the Joker or taking a 'am I a sociopath' test online and finding your new edgelord persona only to find it deeply cringeworthy later.
100 years ago, people with mental illness were assumed to be "possessed by demons" and were either institutionalized against their will, lobotomized, or both. Or just left to die.