Floyd Patterson and Ezzard Charles weren't in good mental condition by the time they were dead. You're going to have a very long list here unfortunately. Holyfield and Mosley are in bad nick yet they're still fighting. They need to stop.
I don't thin Tyson is that bad. He only got beat up really bad three times and he doesn't sound any worse now than he did when he was champ and had barely been tapped. He always had that weird, subtle little speech impediment. The Quarry brothers were/are awful examples. Benitez was bad. Arguello wasn't great. Rocky Lockridge is a mess today, but it's unclear if it's punch drunkness or substance abuse. The Mayweathers are another case of.... is it brain damage or plain stupidity. Meldrick Taylor is a sad case. Riddick Bowe is pretty bad. Holyfield is becoming really bad.
Michael Watson and Gerald McClellan are products of that early 90s, killer SMW division. Watson might've been the best all-around boxer of the whole lot, and turned out the worst.
Charley Burley http://www.youtube.com/watch?v=7BOHIxGiTzA&feature=player_detailpage#t=133s Meldrick Taylor http://www.youtube.com/watch?v=J4fkqBvP244&feature=player_detailpage#t=305s Sugar Ray Robinson http://www.youtube.com/watch?v=Pr17VucMwQY&feature=player_detailpage#t=151s truly sad :-(