You seem to be minimizing if not flat-out dismissing the probability that there could have been fatalities had a gun been used. It's also kind of silly to think that everybody would have the presence of mind to immediately recognize what what happening, turn and run. When one hears a loud, unexpected sharp noise in a hallway, the immediately thought is probably not going to be "GUN!!! RUN!!!". It's more likely going to be "Huh? What was that?". Multiple shots could be fired before everybody in that hallway realizes what the **** is going on.
Also, what the individuals involved think is irrelevant to the overall conversation. They would hardly have objective, non-biased opinions. But if I were to speculate, I would guess that the stab victims are probably thinking "I'm glad he didn't have a gun".
Finally, theoretically, would one dead person be better than 20 injured persons? Is that what we, as a society, should accept as a reasonable trade off?