Boxers Who Killed Themelves, And How

[The Old LefHook]

This one is easy for me--I don't know any, but my boys are smarter than I am..

[Tatabanya]

Edwin Valero, self-hanging (after killing his wife, in that imitating Billy Papke).
Alexis Arguello, self-shooting.

[Tatabanya]

Darren Sutherland, though less remembered, also hanged himself according to the reports.

[Willie Pep 229]

Taking the OP literally - "killed themselves" put Ali and SRR on the list. One laid on the ropes too often, the other fought too long

Both committed long term suicide, and thanks to Bundini Brown we know Ali knew what he was doing to himself.