Your grandma told you it was so: "Idle hands are the Devil's workshop." The story of Tamerlan Tsarnaev, the elder brother of the Boston bombers, lends strong credence to this largely ignored wisdom. In the article linked below, we see that Tsarnaev was apparently lazy before he was evil.
http://www.weeklystandard.com/blogs/tamerlan-tsarnaev-and-family-received-welfare_719056.html