I think that Nicholas fell on top of him and that the walkers were eating him, not Glenn. Lots of people think he'll either be able to hide under the dumpster or cover himself in Nicholas' blood to escape. I hope that's true! It would really kind of suck to have him die with no one knowing about it and spending the rest of the show looking for Glenn. With all of the walkers there I don't think it's likely that they'd find Glenn among them.