Grayback
Since the goal was Dom's slime, I understand him dying to get it. However, Elena's death was both pointless a potentially dangerous. She was asleep. She would have helped prove his story about Dom going back into the cave alone, and thus explain his disappearance easily. Now there's two missing people and he was seen going with one of them. He will have to make a story up about this whole thing and it will be questionable. After all, the longer a lie, the more its likely to have holes.