I just read this story of how vandals opened up and apparently contaminated a reservoir for drinking water. Above than normal levels of mercury were discovered.
This struck me, because this is a type of attack that could really hurt a large amount of people: attacking one of our basic requirements for life and survival.
Mercury isn't really that much an issue really, barring somebody getting a large amount of mercuric chloride, we shouldn't be seeing may fatalities resulting from an attack on our water supply with mercury.
But there are other dangerous chemicals, and better targets to attack with them.
Cyanide salts can be bought with little problem, somebody was found in posession of ricin not long ago, ... the weapons are out there!
I'm sure you can imagine a myriad of "targets" for a criminal who seeks to poison a large amount of people, I think I best not share my personal ideas ... wouldn't want to give anybody any ideas.
Seriously, once criminals get wise, guns will be the least of our worries.