Stabbed in the chest = fatality. After the guy made his first few chest strikes, there was no rescuing the victim from his demise. Sucks, but he was hosed. The driver seems to have got everyone off the bus while managing to keep the freako with the knife trapped inside. Doesn't sound like much else he coulda done.
We all want to be heroes, but sometimes shit just happens so fast you cannot respond in the way you would like too. You hear someone scream from two seats away, you look and ... while the shock of what is registering in your mind delays you a tiny moment ... whoops too late. The victim has a perforated heart and is bleeding out his last moments. More people getting killed wouldn't save the poor victim a bit.
At least it was only one victim and not several at once like is going on over in China.
20 is sooooo young though ... that kid barely had time to know he was alive.