That wasn't the point though. Cho was crazy a mad man. A bullet or two put in him from a professor or student may have saved many lives. Cho was bent on killing people and himself. Every person that man killed deserved the right to self defense.
If the quote is accurate, he made two points. The first is the one I questioned... And that was his point.
The second is the one you came back with. Yes, If someone had been sitting in class with a weapon and reacted properly, maybe fewer would have died.