Here's the thing: Glenn was doomed to die even before this whole ordeal with Nicholas and the dumpster. What's more, his death is one of the most gruesome in the entire comic books series. A new villain named Negan, who is yet to be introduced on the show, bludgeons Glenn to death with a barbed-wire-wrapped baseball bat. The scene is notoriously brutal, barbaric, and graphic, and it's a good marker for exactly how bleak the world is in The Walking Dead. Depicting it on the show would be another strong way to reaffirm the same point, plus to reinforce the show's biggest rule: no one is safe.
There are tons of rumors that Negan will come in before the end of season six. It's worth noting that the show's creators have strayed from the comic books before, so that might mean that someone else will die in this way. Regardless, the fact that Negan has already been cast means someone is on the chopping block, and that someone is very likely Glenn. So, don't get too comfortable with Glenn's return to the show. He might be gone again soon, but at least he's getting the kind of send-off we'd hope he'd get, if he had to die.
