Just thought to throw this lighthearted one out. Two of the most obvious that come to mind are Carlos Monzon and Tyson Fury (although Fury had a couple of doozies).
Jack Johnson. I like reading about him but I could not get myself to rewatch any Johnson fight in full.
That Monzon, what a lighthearted crazy character the way he beat women and murdered at least one. Let me tell you about Charles Manson and Ted Bundy. You talk about characters …