The Devil King's reward is just a poisoned apple.
Think of it as playing an RPG, (let's go with FF7) you're maybe level 15, and an NPC gives you the chance to skip right to the room before the last boss (J.E.N.O.V.A/Sephiroth).
Do you visualize yourself defeating the last boss with your level 15 scrawny ass, no stats, no equipment, and NO PARTY since it's only one person?