Some pillagers get sick of killing and robbing the innocent, causing them to quit. Pillager groups typically just let them leave, as they usually get killed on their own and there's no sense in forcing someone who's disloyal to stick around.
Those who don't get mauled by zombies in the first 24 hours tend to do pretty well. Some become nomads, exploring the world and living on their own. (Sometimes nomads encounter each other and start traveling together.) Others become solo thieves who get their loot with stealth and cunning rather than murder.
In rare cases, they might join a village, although finding a village that will let them in is easier said than done. Some will start living near a village, coming into town to buy groceries and doing odd jobs for emeralds. Those who do this will often find work as scouts and/or cartographers, on account of being a lot more willing to leave safety than most villagers are.
Oh, what a cool concept! I can already imagine all the different stories of outcast illagers, each having their own lives disconnected from the big group. All the possibilities!
