In the TVD Promo/Preview Klaus is asking Stefan if he wants "a drink" from the doppelganger I think he compels him to feed on Elena (once the clock counts down) and not stop until she dies
That's why Stefan's like "I won't be able to stop" Otherwise he's fed from Elena many times before & he's stopped
Klaus gets his hybrids & revenge on Stefan (for lying to him)
What do you think?