6. Glenn should never have been killed off
While we're on the subject of character deaths that never should have happened, we have to talk about Glenn Rhee. Look, we get it, this is an apocalypse show and you should never get too attached to anyone. That includes Glenn, because nobody on a show like this is safe. And yet Glenn should have been - not just because he was one of the best characters on the show, but because the show already toyed with "killing him off" in the previous season.
You remember dumpster-gate? In case you don't, it's the moment when Nicholas shot himself to escape a gruesome death at the hands of the walkers and fell into Glenn, knocking them both off the dumpster they were standing on and into the herd below. When it looked like Glenn was being torn apart, we all thought he was dead. It turns out that he was simply watching Nicholas' body get eaten and, eventually, was able to scramble under the dumpster to safety.
It was a widely criticized moment that many felt was when TWD jumped the shark. But it was made that much worse by the fact that, shortly after Glenn was revealed to be alive, he was one of Negan's mystery victims. Therefore, the show either manipulated the fans and played with ther emotions for the sake of shock value or undercut its long-term stroytelling with a last-minute switcheroo to have Glenn be one of the mystery victims (again, for shock value).
Either way, the violent death for such a beloved character was widely panned and, in spite of the storylines that moment produced for Maggie and Negan in the years after that (that continued on into Dead City), it should never have happened.