Is it bad to act upon nostalgia?

its more like, none of us were ever sane. and i know how me and one other turned out. the leader just has anxiety so bad he barely leaves his house, im so depressed that i barely function. and like he always had psychopath tendancies, idk part of me wants to get back in touch and actually meet up but the other half says he might kill me

This sounds like it's outside the scope of nostalgia and instead you're worried about your safety. Try talking to him online first and gauge how he is. If you still feel he might hurt you then don't go no matter how turned on you feel.