Well you know its one thing to engage someone in a debate in a proper forum and its entirely another when you approach some random people in public who may not be exactly interested in debating you and you shame them if they dont (im not saying thats what happened though).
But honestly, you dont just approach a person who is minding his/her business and instigate an argument with them for whatever reason. Decent people don't do that.
The people who intervened may have seen him as threatening those women specially if he was armed or was wielding a knife and was talking in a very argumentative/aggressive way.
Maybe they decided to put him down as quickly as possible to neutralized his threatening presence but obviously they failed.
My point is the victims of the stabbing could also interpret their position as self-defense.