Learn English – Word/ phrase for satisfaction derived from helping others

He wants to "atone" for past behavior

Definition of "atone" from Google

Verb

Make amends or reparation: "he was being helpful, to atone for his past mistakes".

Synonyms expiate - redeem

EDIT:

Another idiomatic alternative is "to make amends":

From Merriam-Webster Online Dictionary:

Make amends: compensation for a loss or injury : recompense

Origin of AMENDS

Middle English amendes, from Anglo-French, plural of amende reparation, from amender First Known Use: 14th century

Example - step 8 from the 12 steps in a 12-step program:

8. Make a list of all persons we had harmed, and became willing to make amends to them all.

Mudita in Buddhism describes appreciative, sympathetic or vicarious joy—"joy in the greatness of others". I've not seen it used outside Buddhism, but it might make a good coinage.

The article also links to the more English-sounding word compersion, but I can't find it in a dictionary or used in a wider context than polyamory, so its existing meaning may be skewed toward the romantic.