In the first place, human depravity and sin don't surprise we Lutherans. We understand original sin and how those unmoved by the grace of God given in Christ are left disarmed before the temptations to sin created by the devil, the world, and our sinful selves.
Secondly, we understand that because there are people who refuse to live voluntarily under the Kingdom of God ruled by the loving grace of Christ, God has established another means of rule: the kingdom of the world. This kingdom--the kingdom of governments and such--has every right and, in fact, every responsibility to bring the people responsible for the Boston Marathon bombings to justice. This kingdom is meant to act as a bridle on the sinful actions of those whose lives are not being reconstructed by the grace of God in Christ.
Thirdly, we understand that even when the perpetrators of these bombings are apprehended, arrested, tried, and convicted, God will want them, even as they receive the punishment they deserve, to enter His kingdom through repentance and faith in Christ. When Jesus was crucified unjustly, neither the thief on the cross nor Jesus Himself denied that the thief deserved the criminal punishment that the thief received from the civil authorities. But when the thief asked Jesus, in an obvious statement of repentance and belief, to be remembered in Jesus' kingdom, Jesus promised him that on that very day, the thief would be with him in paradise.
Terrorism and murder can't go unpunished. Grace isn't a license to sin. But true justice is not revenge either.