I suppose the trick here is Paladin's questioning. "Did you select the Demon King's reward?" No one selected it, it was given . So I suppose Paladin is the "traitor", though I doubt she'll actually try to kill the top 10. She might though. Though honestly, such a wording trick just puts most suspicion on Paladin so this doesn't really add up.
To those saying the MC was a fool for not trying out Demon King's reward. He made a logical choice (though not necessarily optimal choice). The Flame Emperor had the time travel ability as well and he didn't reach Floor 100. No fking way the guy didn't pick it. As Sword Ghost Teacher said, it's probably a trap. If you've learnt anything from Re:zero (which I assume everyone should know about), it's that trauma doesn't go away even if you revive.
@ Lonewolf22
As
@PantsuKami stated, no I wouldn't classify the guild masters as fools here.
Heresy inquisitor just decided that rather than running the risk of having a traitor, it'd be easiest to just kill the bottom 3. He has no qualms about killing randoms.
Unfortunately he was wrong so that triggered a life and death fight with sword saint, and the other guild masters don't want Heresy Inquisitor dying.