This is well portrayed in the Discovery documentary Everest, which shows a group of people attempting to get to the summit. The mountain is so full of people that are queues of people waiting in line while inexperienced climbers ahead move slowly through the bottle necks (narrow ledges and ladders, etc).
The documentary also portrays the death of climber David Sharp [1] who was left to die up in the mountain. In the documentary, it is shown that he was found during descent in a very bad condition. There is some controversy if this is true and whether he was actually found on the ascent instead. If he was found during the descent, leaving him might be considered acceptable since the party who found him would have been exhausted, low on oxygen and the daylight fading fast.
But if the situation is (as some have suggested, and the Wikipedia article says) that he was actually found on ascent and left to die while going for the summit instead, it is very morally questionable thing to do. Going to the summit, which is essentially only a tourist attraction at this point, instead of attempting to save a man's life is something that I might never comprehend. Even if the chances of saving that man were slim, not attempting the rescue is questionable.