Amazing that so many people are apparently in possession of the full facts and feel at liberty to hand out medals on the basis of something they know absolutely nothing about other than the speculation read in the press and social media, you know, those paragons of truth. 
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There are so many potential factors here. The woman he was allegedly trying to save also died, and it is not yet known how. If she died because of being run over by him then this will complicate matters.
We don’t know the stabber wasn’t known to the man.
We don’t know the potential mental state of the stabber. If for instance he was suffering from a mental episode while his actions should not be excused they would be more easily mitigated on the basis of diminished responsibility.
The law states that you can use “reasonable force” in terms of self defence. In this instance there were multiple people on the pavement. It was possible that he could have hit others in his quest to kill the man.
It’s not unlike a burglary where someone kills the burglar in self defence. If you e.g. happen upon a burglar in your kitchen and pick up a knife from the side and stab him and he dies, then that is reasonable force. if you keep a knife under your pillow and stab him with the knife meant for such a purpose, then it is not, and you would likely be charged and possibly convicted.
But we don’t know any of it. And it’s not up to any of us to decide what should or shouldn’t happen. And while this might not be considered vigilante justice, it’s a slippery slope towards condoning vigilante justice if we decide that this was justified based on the attacker’s actions.