Personally, normally, I would dial 911 first, reach down and feel for a pulse at his corotid/jugular, if still alive I'd see if he was awake and responding... Would stay until the police/paramedics arrive.. Thats where I would stop.. If he required actual first aid, I honestly wouldn't do it... Because I wouldn't be willing to get his blood on me.. This risk is too great, for a person unknown to me!
As far as ppl walking by, and doing absolutely nothing? Many possible factors are listed here already, all perfectly true...
Take a hard look at the world around you, does it surprise you that things are this way? It shouldn't!
Things are just not the way they used to be in days gone by.. I have modified my own behavior because of it, self preservation!
When I leave the small town where I have lived for 16yrs, I consider it all "indian country", and count on noone, only myself..
I never wanted it this way, somehow it just happened...

It's too late to go back to the way it was now..
Damn Shame, the guy deserved better!

RC