Personally speaking, I can recommend living by that excellent, well-proven, and wise motto, attributed to that great genius, Anonymous .....
"Be careful whose toes you step on today - because they might be connected to the foot that kicks your ass tomorrow .... "
Babablackship, it must be saddening to see someone on his deathbed undergoing such regrets and personal ass-kicking, over behaviour he now acknowledges was wrong.
I think we have all been in that position to a certain extent. However, to be in a position where you can ruin anothers career via a derogatory assessment is indeed a very powerful position to be in.
However, the story of the world is of millions of peoples careers being ruined by powerful people exerting powerful forces. Nothing has changed since time began.
Perhaps your best approach to your friends self-flaggellation is to let him know that those whose careers he ruined may not see things the same way he does now - and the passage of time most certainly dulls the edge of deep cuts.
As one whose life was ruined, and whose 30 year asset accumulation was totally destroyed, by one callous person in a position of great power over me - I can assure you that the initial rage of grossly unfair (and illegal) treatment does not usually linger for decades, in the average person.
We move on, and put the poor treatment behind us, and find another life. I'd suggest that you advise your friend, that those he mistreated have also done the same - and possibly do not now regard the poor treatment that they received at his hands, with a continuing aim to exact revenge.
In fact, he may find that some of his poorly-treated underlings may completely shrug off the treatment that they received at his hands, and they may advise him that they went on to better and bigger things, in a different field.