As somebody who's been in a situation similar to this (fortunately they never got as far as pulling the knife because I stopped traffic), I still find it hard to blame people who don't help - you need to know where to draw the line in help, and it's a very blurry line. Your personal experiences are sadly biased; more than one helpful person has ended up dead because they were out of their depth [1][2][3]. As you say, such is life.

[1] http://blog.er24.co.za/2010/11/young-man-killed-in-good-sama...

[2] http://www.iol.co.za/news/crime-courts/good-samaritan-brutal...

[3] http://www.sowetanlive.co.za/sowetan/archive/2007/10/25/warn...