Two news stories from yesterday: In China, a man using a knife attacked schoolchildren. 22 were injured, and none died. Another man in the USA attacked schoolchildren using Sig Sauer and Glock semi-automatic pistols, precision instruments designed to efficiently kill humans. There were almost no survivors. Apparently, every single child he targeted was killed on the scene. The local hospital turned away nurses who had come in to help, since there was no more help to give.
Even for a determined murderer, killing someone with a knife is difficult. You'll generally need to hit them several times, and if you don't hit a major artery, they are likely to survive, especially with modern medical treatment. Plus, they will defend themselves, if they're conscious. Killing someone with a bomb is also a challenge -- as recent failed bombings both in the US and in Germany (g) show, it's very hard to competently construct a large bomb.
A semi-automatic pistol is a different matter entirely. Standing well away from your victim, you can launch projectiles at 1,150 feet per second at their head and chest, pumping bullet after bullet into them until you're sure they're dead. The whole process takes seconds. There's almost nothing they can do to defend themselves. You can stay a comfortable distance away from your victims. Plus, you can pick them out specifically and target only the ones you want to kill. The gun that gives you this power is light and easily-concealed.
This is why most societies tightly regulate semi-automatic handguns. There's nothing like them for the combination of easy concealment and potent lethality. There will always be the occasional extremely rare incident in which one person decides to attack a large number of others at once, but only if he uses a semi-automatic weapon is the death toll likely to be high.
This latest American massacre will likely spur more debate about gun control in the US, but gun-control opponents there will rightly point out that tighter regulations probably won't achieve much. The genie of portable, effective killing machines is out of the bottle: the US is awash in 310 million non-military privately-held firearms, millions of which are semi-automatic handguns. There's nothing you can do to take them out of circulation -- at least nothing that is remotely politically feasible. The US will be living with this policy disaster for decades, if not centuries.
(Graph source here)