And while the first guy would probably get stabbed and maybe killed, the others behind him would likely survive.
So, if you want to be first, I'll follow right behind you.
By the time you figure out what is happening, that passenger may have been stabbed multiple times. You have to weigh the chance of saving them with the likelyhood you are going to get killed trying.
The main reason for intervening at that time would be to prevent anybody else from getting hurt. If everybody else is getting off the bus, and nobody is left to be killed, you would be risking your life to stop him after he probably already killed the guy.
So I don't fault anybody for fleeing. If he was trying to stab the bus driver, then I would have intervened, or if it was on an AIRPLANE where he might try to take over the plane.
BTW, an ex-military guy still might be no match against another man who has martial arts training and a knife.
Grab a suitcase from an overhead rack and hold it in front of you while running towards him. No need to run straight into the knife (thought the perp was so intent on stabbing his randomly chosen victim, that it’s not clear he’d have bothered to turn around if someone had been coming at him). And actually, a couple of people throwing suitcases and duffle bags at the guy should have been able to get him down on the ground.