That's surprising to me. Goblin Slayer is powerful, I'm sure, but we've already seen that he's a man with emotions. He can be short with his subordinates, exasperated by incompetence, and just comes across as a dangerous but otherwise normal guy. (I don't think he's insane at all. Sadistic, yes, but he's far from delusional.)
In other words, Goblin Slayer can get frustrated. He can fumble. No matter how nasty he gets, you know he puts his pants on one leg at a time and occasionally stubs his toe just like the rest of us. If I was trapped in a room with him, that knowledge would put me on more of a level plane with him in my own mind, and perhaps give me the moxy to fight him (disregarding for the moment that he's a level ?? Ranger.)
Kore, on the other hand.... Kore doesn't come across to me as anything besides death incarnate. Just seeing him coming at you is an incredibly disheartening experience. So far we haven't seen him get ruffled for even a second in the comic, and that gives him an air of invincibility that's much more terrifying to me than simple human sadism.