وَلَا تَقْتُلُوا النَّفْسَ الَّتِيْ حَرَّمَ اللّٰهُ اِلَّا بِالْحَقِّۗ وَمَنْ قُتِلَ مَظْلُوْمًا فَقَدْ جَعَلْنَا لِوَلِيِّهٖ سُلْطٰنًا فَلَا يُسْرِفْ فِّى الْقَتْلِۗ اِنَّهٗ كَانَ مَنْصُوْرًا ( الإسراء: ٣٣ )
Wa laa taqtulun nafsal latee harramal laahu illaa bilhaqq; wa man qutila mazlooman faqad ja'alnaa liwaliyyihee sultaanan falaa yusrif fil qatli innahoo kaana mansooraa (al-ʾIsrāʾ 17:33)
And do not kill the soul [i.e., person] which Allah has forbidden, except by right. And whoever is killed unjustly – We have given his heir authority, but let him not exceed limits in [the matter of] taking life. Indeed, he has been supported [by the law]. (Al-Isra  : 33)
1 Mufti Taqi Usmani
Do not kill any person the life of whom is sanctified by Allah, except for a just reason. And whoever is killed unjustly, We have invested his heir with authority (of equal retaliation), but he must not cross the limit in the matter of killing. Surely, he will be helped.