From the reports I have seen it doesn't look as if he had it in for the Amish particularly. The school was simply there, and it offered ready hostages/victims, that's all.

As for the gun thing: To commit a crime you need a motive and opportunity. Reducing the availability of weapons reduces the opportunity, plain and simple, no stats or convoluted explanations needed. As a corollary, it would also reduce the number of gun-related accidents, for the same reason (reduced opportuity).

As for 'rights', we are born with no rights, not a single one, except those that society and laws confer. The right to bear arms is one such, in some countries; but it's society's choice to confer it, not a god-given right.