The face of marshal

Ok, they caught the mad bombers in Mass.the last one a 19 year old college student. I really don't under stand why a person would set out to main and kill people they don't know or someone they have no reason other than they can do so .
I did learn something while I watched  on TV. What we were looking at in the town was marshal law. all those car we saw on the streets were law mans cars either city, state or federal law men were driving the cars and trucks because everyone else had been ordered to stay inside.

Manhunt: A massive manhunt is underway in the greater Boston area to try and locate Tamerlan's younger brother Dzhokhar, who is still at large