Any of the Triad, Tyr, Torm and Ilmater, would be fine. There is a good prestige class for followers of Ilmater in PFtF, Martyred Champion of Ilmater, that gives monk-like unarmed combat abilities which may be preferable to actually "wasting" XP on actual monk levels.
Another interesting possibly is Eldath whose portfolio includes peace. The PC's "vendetta" against the undead isn't so much about slaying them as granting them eternal peace.
I would also include Lathander as a possibility.
If you're prepared to go further afield for your choice of power, I would also consider Osiris or Horus-Re from the Mulhorandi pantheon. There's a particularly strong prestige class in PGtF for followers of Horus-Re, Eye of Horus-Re, in PGtF and also on the WotC website.