Han Sen thought of many different things he could wish for. He thought of wishing for God to be dead or for Han Sen himself to become God. Both of those seemed like wishes that God would be unable to fulfill, and thus would be forced to renege on his bargain.
But when Han Sen thought about it some more, he realized that those wishes wouldn't work. If Han Sen wished for God to die, he needed a way to make sure that God really was dead.
Ordinarily, death meant losing all signs of life. But different types of life also had different types of life signs.
When Han Sen first entered the seventh floor and saw the man, he thought the man was dead. If that was the "death" Han Sen was referring to, then God would fulfill his wish without difficulty. He could make himself lose all signs of life without being negatively impacted.