Name the wrong
Deliberately taking a human life; assisting a credible plan contributes to grave danger.
Commandment 6 · Value human life

A long dispute has hardened into revenge. Someone you know says they intend to kill the other person and describes a credible plan. They ask you to help locate them. You know the history, and part of you wants the offender to be frightened. But sympathy with a grievance cannot justify taking a human life.
Thou shalt not kill.Exodus 20:13 · King James Version
Choose the response you are prepared to defend. Then examine the cost.
Deliberately taking a human life; assisting a credible plan contributes to grave danger.
Valuing life enough to refuse participation and seek protection.
The command here is not a metaphor for annoyance. This case concerns a real threat of killing and a responsibility to seek help.
Examine the sentence “They deserve it.” Has it become permission to dehumanise someone? If the case resembles a real immediate threat, stop reading and contact appropriate emergency help.