So the real reason they killed Carl is
Because Chandler Riggs is turning 18 and they didn't wanna pay him more as an adult?
I just heard about this and i absolutely 100% believe it, this show is all about saving as much money as possible, which is why they have episodes and don't show everyone but just a few and show the others in another episode, so they don't have to pay everyone for every episode. Which is why they always have to find an excuse for the group to be split up every season.
And i find it funny right before Chandler is supposed to turn 18 they decide to kill his character off, a character who is extremely important in the comics who takes Rick's place, he's essentially the John Connor of Walking Dead. So right before he turns 18 and will then have to be paid as an adult which is more money, they conveniently think its best for the story to kill Carl.
If i was Chandler i'd be pissed and spread the word about what they did.