Evil people

I think the bombings in London are so tragic. Especially just when everyone was so happy celebrating their win to host the 2012 Olympics. We were watching TV while having dinner and discovered that this horrible thing happened. How evil can people be? We are all humans with families we just want to go back to everyday after work. How can people do this to each other? How do people who plan these things go home to their families each day, have dinner or play with their kids?