I'm writing a story and I'm a bit stuck with a detail in the plot, I'm hoping someone can help me to get rolling again.

Two characters are jointly involved in ongoing medical malpractice, 'Doctor Evil' because he is evil, 'sidekick' because he has no choice, 'Doctor Evil' is blackmailing him to do it.

'Doctor Evil' needs to have a pretty big hold over 'sidekick' in order to get him to cooperate, I need to establish what that thing is.

If 'Doctor Evil' once saved 'sidekick' from certain death, would that be enough? I mean, if you owe someone your life, does that mean they have a degree of control over you forever? Or would there come a point where you say "thanks for saving my life, but I'm not doing this evil stuff anymore"?

I have an alternative plot option where 'sidekick' is even more evil than the doctor and he's doing the blackmailing. He has evidence of the doctor's past which the doctor must keep hidden. Unfortunately this storyline leads to some complications so I'd rather go with the first one.

Any comments would be most appreciated.