Could it be that the time he has spent as undead is what has returned him to Lawful Good?
The way I could see this easily retconned.
Paladin sent to help a village out with a church (not Ilmater, they wouldn't approve of mercy killing ever)
Necromancer conspires to destroy him, make him her prize
Targets village, all suffer for his presence
Repeated disruption of his activities, where ever he goes
A moment of weakness lead him to break Paladin code
Necro bests him
Despair, but also regret
Last ounce of strength allows him to keep his senses, but is forever haunted by zombification and his wrong doings
Has spent years/decades trying to atone for his sins, make peace with himself.
All that needs doing changing the Deity and the rest can be put down to his seriously guilty conscience not allowing him to mince his words about what hes done. Don't even have to retcon the reactionary posts by others.
So far his actions have only been Lawful Good, what he's said he's done however isn't.