1. The attacker is a recent (~4 years) immigrant to Canada according to the latest news reports. Mental illness can be late onset (usually triggered but a life-changing situation or event, such as immigration), and remain undetected unless close friends and family can recognize the signs and demand help. As a person living within an immigrant enclave, that is exceedingly unlikely.
2. Greyhound bus corridors are less than 2 feet wide, and the guy is wielding a large knife and already demonstrated the will to use it viciously. Everyone on board was exactly right not to intervene - you get the hell out of harm's way and let the police deal with it. Unless you know how to deal with close quarters knife fights and are right next to him, you have precisely ZERO chance of saving the victim and a really good chance of getting stabbed yourself.
3. Guns. *sigh*. First off, all weapons are not allowed to be carried on a bus. Handguns are perfectly legal in Canada as long as the proper procedures are followed (or at least most) as are long guns. However, no one would be carrying on a bus, and certainly not openly. Not to mention, unless you are police or active-service military trained you are much more likely to hit someone else in that cramped environment than the guy you're aiming for. This is a bus, not an empty street at high noon thanks. Don't be ridiculous.
4. Whether or not a gun was trained on the guy its pretty clear there is mental illness involved. Considering the victim was probably dead within the first few blows and how quickly they can be executed, the guy isn't stopping in time and probably not stopping at all. Use your brain - simplistic bravado is exactly what a proper analysis of this situation DOESN'T need.
As an aside, compare the violent crime rate in the town in that gun town USA article to the violent crime rate in a similar sized town in Canada and tell me what you see. I'm not telling you to look at it because of different philosophies on gun control (you can't; the underlying cultures are too different), but rather to compare similar cultures which differ on their attitudes toward guns and violence in general.