As in this case. Christians need to stop speaking nonsense, especially to those whom we seek to win for Christ. It’s not a binary choice. In the civil kingdom, a kingdom of law and justice, where grace and love, to quote John Cleese, “don’t enter into it.” Terrorists ought to receive a full and just punishment.
Christians, however, live simultaneously in another kingdom and we may speak of grace and love in that kingdom. We should preach the law and the gospel to terrorists and to all other sinners and, should they turn from sin and embrace Christ heartily, we should embrace them and forgive them, entrance into that kingdom doesn’t change their civil obligations.
It’s not an “either/or” question. It’s a “both/and” question.