Although were I running the game, letting Kore cast would be a not-fun option. But that's just because of my group - enough of us know the rules well enough to expect that Hold Person would lock him down unless he made the save, so breaking that expectation to let my antagonist keep on truckin' would've been shutting down player agency (especially since, if Kore is a paladin, relying on a failed save is a huge, huge gamble which should be rewarded, if anything).
That's something weird I noticed: Paladins don't actually have good will saves. If Kore is level 10, his will save is a mere +3 (plus wisdom modifier, plus *charisma* modifier because of "divine grace" class ability). That's likely to be around a 10 assuming his charisma and wisdom stats are actually high. Forgath's hold person spell would have a DC of 12 plus his wisdom, so probably at least 15. That's roughly a 25% chance of success, not too shabby against a target *6 levels above him*.
Of course, Hold Person has apparently been nerfed in this universe, but Forgath prepared it without expecting a divine spellcaster.
About whether Forgath and Minmax could have avoided Kore's ire: I don't think so. The way Forgath reacted, I suspect Kore kills *everyone*. I don't know what excuse he'd find to kill Forgath, but when he's willing to kill a young child for spending a few months around monsters, he can probably think of something.
Keep in mind he has Detect Evil but doesn't use it. He doesn't care if someone's evil right now, just if they've been exposed to evil and thus might, someday, do evil.
http://www.goblinscomic.org/12122013/"Wait, Kore? You mean like, THE Kore? The cursed scourge of the realm? Last of the Greyhill Paladins? THAT Kore? Oh no."