I realize that many species have a well-deserved reputation for being savage but (so far) the meanest individual I have yet encountered was a young male Electric Blue Acara. They are supposed to be mellow and indeed most are.
But this guy... in one night he killed an angelfish and mangled its mate, he slaughtered the German blue rams, he pulled the eyeballs out of all my Geophagus sp. Red Head Tapajos and savaged his would be girlfriend.
I named him Jack the Ripper and he got his own tank. Karma caught up with him though and he jumped out to his death before I found a lid.
Just saying... the heavy hitters have earned their reps, but sometimes you can get an individual from a "peaceful" species which simply has murder in its evil little heart. It's all the more shocking and distressing because you don't see it coming until it's too late.