Sarcasm is an adjusted stress reaction.

Seriously, I don't get people. I figured out William would die in the war at the end of last series; Grantham is too old to go to war, Bates too injured, Thomas has too much of a role as mustache-twirling villian to be killed off, they won't kill the heir (seriously maim, yes; kill, no), and that only left William, who had the sweetheart to make it all the more poignant. Okay, so I'd forgotten about Branson and Molesley, but I always forgot about Molesley last series, and it just didn't occur to me that Branson would be in the right age group etc, too. But anyway, I was right in my logic!

tl;dr: William was always going to be the character they'd kill off for emotional impact.