There's a lot of blame to go around this year and I'd argue that if you insist on blaming only one person you're letting a lot of guys get off without taking responsibility for their failure. That said, this is America and we pretty much need to decide whose fault this is even if that's not fair or accurate.
There's a strong argument that the entire culture of the San Diego Chargers is the problem and I think it's fair to say that starts at the top. Dean Spanos, who is probably just counting the days until he can sell the franchise and get as far away from football as he can, has made a habit of making bad situations worse by letting them fester. He's also the man that hired A.J. Smith and signed off on the hiring of Norv Turner, so if you blame them it's hard not to just blame Dean.
A.J. Smith has had a pretty good run as the General Manager of the San Diego Chargers in all honesty, but this train started derailing years ago and he hasn't managed to get it back on the tracks yet. Failed draft picks in the middle of his tenure and horribly failed free agents this past season have left the team lacking in both depth and impact starters. He's also the man that hired Norv Turner, so if you blame Norv it's pretty hard not to just blame A.J.
Norv Turner's hiring was universally hated the day it occurred. He responded by having greater success than any previous Chargers head coach ever even as the overall talent level he had to work with fell with each passing season. Some of you don't see it that way. You think he was gifted the greatest roster of all time every year and the only reason the team didn't win the Super Bowl each season is because of Norv Turner. That's stupid and you're stupid for thinking it, but it'll still be an option in the poll.
Then there are the players. Maybe if some guys had just stayed healthy or just not made that one boneheaded play in that one game things could have been different. Ultimately the games are played on the field and I'd be remiss if I didn't give you the option of putting the blame squarely on those men on the field.