Any alignment can join any faction; it's the faction goal that matters! I would also vote against Osirion, as this character seems to want to do what's good and right, which fits Silver Crusade. But take a look at the faction goals and decide what best aligns with your character's interest. Keep in mind though that some of the factions are changing for season 6!
The thing about Silver Crusade is that the character has strong opinions about undead, but other than that is mostly indifferent to good and evil. He probably would support the Mendevian crusades for the same reason he opposes undead; the worldwound is "unnatural" and poses a threat not just to mortal beings but to the natural order itself.
But he doesn't really care about ordinary people being killed or enslaved; in his mind Pharasma will gather them all eventually. He is not especially interested in helping those who can't help themselves, at least not without reward. So Silver Crusade feels a little weird for me.