Celebrim
Legend
The only people who will change are the ones who are disingenuous jerks.
I would totally support a "Come to the Shepherd God" moment in a tRPG. Repentance, maturation, wisdom, becoming disillusioned with ones former cynicism are all great story lines. I just never see them come up. No one ever plays a character that way.
One reason why is that IME, only about 20% of players are capable of playing anything but themselves. Their characters have their same beliefs and morality and so to really see those characters change, you'd have to have a real change in the person as well - which is perhaps too much for a mere vain leisure activity to aspire to.
Of the 20% or so that can play something other than themselves, those players most the time are wanting to explore some personality or ideology at length and don't want to just give up on the idea at the first opportunity. Unlike in media, the vast majority of people who want to play a paladin really want to try to successfully be Steve Rogers and win the admiration of all by their grace and honor and courage and gentle warrior virtue. Likewise, if they want to play a rogue then they don't just envision themselves being a confused rogue that in a heartbeat is going to change their character. So ultimately of the 20% that are roleplaying maybe 20% think of their character in terms of a narrative and only some tiny percentage of those think of the narrative they want to construct as being having a major change at some point in their character. Rather they typically have a goal to accomplish as their narrative - reclaim their families honor or land, wed and found a house, get rich, discover the secret of their birth, etc. Conservatively I'd bet you'd have to go through several hundred characters to get a player that wanted the heel-face turn story arc and was capable of pulling it off, or who in the course of play realized it would be cool to pull a heel-face turn based on events that occurred. Believe me, I've tried, but while I can sometimes coax a good and heroic act out of a player who isn't predispositioned to do so by making them care for an NPC or seeing the profit in being a useful ally to someone, rarely does it stick as new belief system, because really it's just them playing themselves.
As for what is rewarding, it very much depends on the aesthetics of play of the particular player. Some players are playing almost entirely to solve puzzles and win. Other players are playing to receive the adulation of their peers for doing something cool. Only a few players are really primarily in it for the story and in particular in it for a story as self-expression that they create as opposed to the experience of watching a story unfold the way they watch a TV show. Exactly how you induce a player to do something with rewards depends on the player. But a player that conceived of his character being part of a heel-face turn story doesn't need a bribe to do that, just a plausible series of events that lead to it which I would happily arrange as subtly and writerly as I could. (Tony Stark or Hon Solo being decent examples of gradual transformations to less and less self-centered and more and more benevolent personalities.) That isn't to say that there might not be some rewards for being good along the way. I'd happily bribe someone who had moved from CN to CG by being actively participating in furthering the weal of others with XP as well. It's just... I don't see it happen.