I can't believe people would see someone collapsing right in front of them and not stop. PB or not, 100 yards from the finish or not, winning the bloody race outright or not, FFS how could anyone just run past someone in trouble?
Different if you see someone who obviously got into trouble a while ago and is already being attended to by marshals and/or paramedics - there's most likely nothing more to help you could do. But if someone in the same race as me, just a few yards ahead of me, crumpled right in front of me? I'd likely want to stay with them till the ambulance arrived. It might sound corny to the ruthless PB hunters, but when I'm running, I feel a sense of kinship with the other people running round about me. I wouldn't run past anyone who was potentially in trouble without stopping first to check that they were okay.
The UTMB has a rule that requires all competitors to give assistance to other runners who might require it in an emergency. Even to the point of a runner having to abandon their own race and backtrack to the last checkpoint. Failure to comply with this rule is an automatic DQ. When I'm running, I have a kind of personal rule like that for myself.
And yes, I also stop for chumps looking for Tesco, annoying dimwits hillwalking without maps, people who want directions even when I have no knowledge of the locale, kids and little old ladies walking their dogs who just want to say hello. If I had to pick between the two, I'd rather be a nice person than a better runner.