Jack Merritt, my classmate at Cambridge and a program participant, recited a poem he’d written on forgiveness and rehabilitation. I remember feeling surprised by his words, so unselfconsciously earnest. I’d known him mostly for his dry wit, his boyishly good looks and his reputation as a criminology class clown. Jack was stabbed to death during a terrorist attack near London Bridge on Friday. The attacker, Usman Khan, had been convicted of terrorism offenses in 2012. He was released from prison last year. Just before the attack, Jack was in Fishmongers’ Hall, a building adjacent to the bridge, helping to lead...