Are you really saying you wouldn't help someone drowning or in danger of being run over, would you attempt to save a child from a savaging by a dog? keep walking if you hear a woman's screams? really i think its out of the ordinary to not act in such circumstances.
I've been surprised by how much we're subject to herd mentality. Every person who doesn't stop to help makes it more likely that the next person won't help either.
A few years back, I came across a car on fire on the side of the road. I stopped to help, but the fire was already too big - I dumped my whole fire extinguisher into it, but it wasn't enough to actually put it out.
As I stood there with nothing else to put out the fire, just waiting for the fire department to arrive,
everybody drove by. Not one person stopped to help or even to see that everyone was all right until an off-duty fire chief did stop... but by that point, the fire was way beyond putting out with handheld extinguishers.
When I had first arrived, it was a moderate engine fire. Serious, but not so severe that the car would be a write-off. By the time the fire trucks arrived, the car was destroyed.
And it wasn't because we had a lack of people that could help - this was on one of the busiest highways in Canada.
Thousands of people drove by that fire and only two people stopped to help.