Moral question on skillful Dharma: Should you withhold assistance that prevents an evil person from dying?

Suppose John Doe is an evil person, 

actively doing evil that harms people presently and in the future. 

You're not a medical doctor, not any kind of licensed medical professional, 

with no legal obligation to help anyone.

But you have healing skills, or knowledge of healing modalities, 

or know of a competent healer who can heal John Doe, 

who is currently under care of healers who can't and don't know how to help him.

John Doe is gravely ill, old, evil.

The sooner he dies, the sooner he can stop harming people in his life. 

What is the best thing to do? 

A friend I know was in this situation,

helped out the John Doe, who survived, is now healthy, and likely to live for many more years

doing harm in the world.

My opinion,

there's no right answer, 

you have to go case by case.

My friend has doubts and regrets, thinking they have caused harm to John Doe's current victims since he helped John Doe to live.