The Cow - 2:178
Believers! Equality in killing is enjoined upon you. Free for free, slave for slave, woman for woman. But whoever is forgiven by the relatives of the deceased, then he must do what is fitting and pay the dowry well. This is a mitigation and mercy from your Lord. Then whoever transgresses after that, for him is a painful torment.
Do not exceed the limit in killing out of revenge
The Night Journey/17:33
Do not kill unjustly a soul which Allah has forbidden. Whoever is killed unjustly, We have authorized his heirs. Let him not exceed the limit in killing out of revenge, for he has been helped