One day in 2006, Paul Wagner donated one of his kidneys to a stranger with kidney failure. Not long before, he had been reading the paper on his lunch break at a Philadelphia company and saw an article about kidney donation. He clicked on the website and almost immediately decided to donate.

One day in 2008, Scott Johnson was sitting by a river in Michigan, feeling aggrieved at the world. He took out a gun and killed three teenagers who were out for a swim. He showed no remorse or guilt—instead, he talked about how other...