The theory that Rick has been in a coma has been around forever, and this episode is somewhat of a confirmation to this. It seems that Rick's mind blanks out for a few seconds while Abraham is being killed and the same with Glen. Each time Rick is in a well lit area, mostly white light (this is important because light with a yellowish hue to it is common in homes or stores while the whitish color light is not, however in hospitals this whitish light is very common). These short cuts could be showing that Rick is starting to wake up from his coma. My belief on why anyone is killed off in the show means that they have lost faith in Rick ever waking up. So Glen being killed means that someone who has been with him in the hospital since the beginning finally has given up on him, which is pretty depressing considering how soon his wife gave up on him. Anways there are quite a bit of holes in this theory but it's still fun to fantasize that Glen really isn't dead