hey "pimp", just relax. What can you say about the situation? Noone wakes up and says "I'm going to ignore all murders I see today, because I don't give 2 shits". If I saw a person getting BEHEADED and STABBED I can't honestly say I would rush to save him. I would first consider my own safety. Secondly, how do you KNOW he was only concentrated on that? You just assume his back was to everyone and they all ran out the other way, who knows.
Seriously, get off your high horse Rambo. Some bad things just happen to good people. Some things you just have to accept happen and can't be changed much. Even if you were to stop the guy IMMEDIATELY after stab #1, who knows if it would even save him? Sure, no one tried, but YOU DON'T KNOW what the situation was like. I can't imagine myself trying to overpower a guy with a "Rambo like" knife while on a bus, enclosed. What's to stop him from stabbing the shit out of me and already having killed the first victim? Theoretically, nothing.
So just relax that a group of 40 people weren't heroes on this day. Bad things happen to good people sometimes. It is a tragedy for sure, but the sun does rise tomorrow.