God has made life sacred. Unjustified killing is totally prohibited.
[17:33] You shall not kill any person - for GOD has made life sacred - except in the course of justice. If one is killed unjustly, then we give his heir authority to enforce justice. Thus, he shall not exceed the limits in avenging the murder; he will be helped
[5:32] Because of this, we decreed for the Children of Israel that anyone who murders any person who had not committed murder or horrendous crimes, it shall be as if he murdered all the people. And anyone who spares a life, it shall be as if he spared the lives of all the people. Our messengers went to them with clear proofs and revelations, but most of them, after all this, are still transgressing.
Killing may be permitted only when people wage war against you.
[2:191] You may kill those who wage war against you, and you may evict them whence they evicted you. Oppression is worse than murder. Do not fight them at the Sacred Masjid, unless they attack you therein. If they attack you, you may kill them. This is the just retribution for those disbelievers.
The justice must be served:
[4:135] O you who believe, you shall be absolutely equitable, and observe GOD, when you serve as witnesses, even against yourselves, or your parents, or your relatives. Whether the accused is rich or poor, GOD takes care of both. Therefore, do not be biased by your personal wishes. If you deviate or disregard (this commandment), then GOD is fully Cognizant of everything you do.
[5:8] O you who believe, you shall be absolutely equitable, and observe GOD, when you serve as witnesses. Do not be provoked by your conflicts with some people into committing injustice. You shall be absolutely equitable, for it is more righteous. You shall observe GOD. GOD is fully Cognizant of everything you do.
The Quranic law of equivalence is a life saving law.
[2:178] O you who believe, equivalence is the law decreed for you when dealing with murder - the free for the free, the slave for the slave, the female for the female. If one is pardoned by the victim's kin, an appreciative response is in order, and an equitable compensation shall be paid. This is an alleviation from your Lord and mercy. Anyone who transgresses beyond this incurs a painful retribution.
[2:179] Equivalence is a life saving law for you, O you who possess intelligence, that you may be righteous.
[42:40] Although the just requital for an injustice is an equivalent retribution, those who pardon and maintain righteousness are rewarded by GOD. He does not love the unjust.
More Quranic wisdom on dealing with murder.
[4:92] No believer shall kill another believer, unless it is an accident. If one kills a believer by accident, he shall atone by freeing a believing slave, and paying a compensation to the victim's family, unless they forfeit such a compensation as a charity. If the victim belonged to people who are at war with you, though he was a believer, you shall atone by freeing a believing slave. If he belonged to people with whom you have signed a peace treaty, you shall pay the compensation in addition to freeing a believing slave. If you cannot find* a slave to free, you shall atone by fasting two consecutive months, in order to be redeemed by GOD. GOD is Knower, Most Wise.
[4:93] Anyone who kills a believer on purpose, his retribution is Hell, wherein he abides forever, GOD is angry with him, and condemns him, and has prepared for him a terrible retribution.
Thank you and may God guide me,