So Rick and Carl are of limits, the creator won't let them touch them. Glen and Michonne are also two of those characters. They are strong characters important to the story that are off limits. Andreas character was screwed up since season two and the fans hated her so they had no choice but to kill her off. Andreas character in the story wasn't as important but she was the groups go to shooter. Michonne is this strong independent character that often times serves as Ricks moral compass, in some ways shes a more badass black female Daryl. They won't kill of Carl because to quote Negan “I can't kill you before your story ends. Too [expletive] interesting.” Glenns demise is written in the stars so they won't stray too far from that. The writers wanted to kill of Daryl and Judith but AMC wouldn't let them because Daryl is too much a fan favorite and they need to milk it and Judith is a baby so its kinda dark which might upset people.
