I actually think they CAN'T kill him. Wikipedia says (ino, ino):

"Like many of the characters on the series, his name has its origins in ancient Greece, Agathon being the name of an Athenian tragic poet. The Greek proverbial phrase kalon k'agathon ("the beautiful and the good") — also written as kalos k'agathos — was used to describe the ideal man, the epitome of both aesthetics and ethics."

SO IF THEY KILLED HIM OFF THEY'D BE KILLING OFF THE IDEAL. DDDDDD: lol I'm so involved it's not even funny