Completed your mission? it means that you will go home and you have nothing more to do. Why prime ministers and Presidents do not retire/quit jobs when reaching 60 of their ages (retirement age in most countries), are they made of different cloth? May be if you go home some other one (which I doubt) would replace you and conclude peace with Palestinians.
NATO backs Turkey's fight against ISIS (Reuters)
from the article: Netanyahu: Shalit's release means I have accomplished my mission