If you need an argument to justify the looting try this:
If you came across an evil warlord who had a magic sceptre that was controlling the minds of his innocent subjects, the dutiful paladin would vanquish the warlord and take the sceptre. The sceptre is a dangerous resource that needs to be kept out of the hands of Evil. Now replace the scpetre with the horde of loot in your fiend-worshipping temple. It's a financial resource, instead of a magical one, but still potentially dangerous in the wrong hands.
It definitely comes down to a question of what he does with the loot afterwards. If he tracks down the original owners of the cult's ill-gotten wealth and returns it to them, that's undoubtably good. If he buys humself a new suit of armor to carry on the good fight more effectively that's probably OK, but is getting into questionable territory. If he buys himself a new summer home by the ocean, he's lookin' at an Atonement in the near future...