Growing up hearing the tale of how he came to be, he decided to become a paladin, to avenge wicked deeds and dedicate himself to the cause of justice. Not so much a vigilante, but to get the guys that slip outside of the law; capturing them when possible and slaying them when not.
So, with that backstory laid out, what god from the standard D&D set would seem to be most appropriate? I think Heironeous is a bit played out as the god of all paladins, so who would be the next best choice for this character concept? I don’t even mind going to a nonhuman god like Bahamut, Mordain, or Yondalla if those will get the job done. Pelor is popular among the party, but I don't know if that god is LG (but if Pelor takes LG characters, that could work just fine).