To be honest I really don't think Glenn is dead. First off, it's a terrible ending for a character who has been at with the group since the
very
beginning. Secondly, if anyone has watched the behind the scenes videos AMC usually posts, the cast and crew go over Glenn's death like it was
NOTHING.
If you watch the scene again, the camera angles look odd, and the we don't get to see a confirmed death. I believe that the walkers were eating Nicholas and Glenn will realize this and quickly hide under the dumpster.