Q n A : He wants to kill his wife’s relatives after they robbed him and took his wife by deceit

Praise be to Allah.If her brother was her
guardian, then the marriage is valid and they have no right to take your
wife away from you. This is wrongdoing on their part. They had no right to
harass you in order to take money from you. The wealth of a Muslim is not
permissible to anyone unless he gives it willingly. 
You have to go to the
sharee’ah courts and turn to the scholars and prominent figures in order to
resolve this problem. 
With regard to killing
him and killing any member of his family, all of that, as you mention in
your question, is a major sin that is worse than blowing up the Ka’bah. 
Whoever does that, the
curse of Allaah, the angels and all the people will be upon him, and his
recompense will be Hell, to abide therein for eternity. 
We ask Allaah to keep us
safe and sound, and to set the affairs of the Muslims straight.