It's no worse than what they were willing to do to the driver.

I think it's pretty presumptious to assume that having a weapon means that they are willing to kill the bus driver. If they actually were, they probably would've shot him, instead of leaving, I'd argue.

Plenty of people will rob others at gunpoint that wouldn't actually be able to pull the trigger. For all we know, that gun might just as well be fake.

We don't need to assume anything here, we have the footage. The robbers were leaving, the guy stabbed one in the back purely for revenge.

To summarize once again: I consider it to be wrong to potentially lethaly injure people that are running away.

It's not preemptive if they've already threatened people's lives.

So just "kill all criminals"?