Violent videogames do not make non-violent people become violent. Just like watching an intricately detailed movie about rape or murder won't make somebody suddenly want to try it. You have to have a substantially weak psyche to be influenced in such a manner. Otherwise everyone watching the news reports would go do the same. The guy was messed up to begin with.
As for arming teachers, there are some serious cons to consider with the pros of it. What happens when the teacher is having a bad day, is forewarned they will be fired or is tired of dealing with that little asshole in the back? Assuming the weapons are kept secure; locked up, trigger locks and ammo stored and locked separately, how do you have the weapon ready for defense in time? A psycho won't care if there is a gun and you need to be ready to act quickly. Lastly, if the guns are not kept secure, what's to stop an abused or bullied student from gaining access?
It is a tragedy what happened. But making rash decisions will only make it worse. Hopefully, cooler heads will prevail.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."