Danny Costello is a a former 3 time ABA champion who was destined for the big time in the pro ranks but found Heroin and Speed which cost him his licence to box. I know Danny from the start of his pro career when Munro/Hyland Brothers signed him. When he went off the rails aI lost contact with him but last year our paths crossed again. Danny has now turned his life back round onto the straight and narrow and is joint running a local youth club in Liverpool training young kids how to box but also making a difference in the community. Here is a link to an article on him and the youth club http://www.myboxingcoach.com/amateur-boxing-changing-lives/
Ricardo Williams - Olympic silver medalist, didn't have his head screwed on when turning pro and eventually ended up serving time for drug trafficking Surprised nobody has mentioned Ibeabuchi Ricky Womack beat Holyfield twice as an amateur, never lost as a pro, got jailed for armed robbery then killed himself.
Bit like Valero, he won world titles though so doesn't really apply. Nonetheless he could have gone on to be an absolute legend