What is the decree of killing innocent in Islam?

The Holy Quran clearly prohibits killing any person; it announces Nor take life - which Allah has made sacred - except for just cause. (Al-Israa Surah, 17:32)
Causes that read in the verse are the situations like:
-warfare
-self-defense
-execution of somebody by the state when his guilt is confirmed.
Apart from these occasional events, to kill somebody is one of the greatest sins. In another verse, the Quran announces:
If anyone slew a person - unless it be for murder or for spreading mischief in the land - it would be as if he slew the whole people: and if any one saved a life, it would be as if he saved the life of the whole people. (Al-Maidah Surah, 5:32)
That is to say, to kill an innocent person is a crime as horrible as killing the whole people. In the verse, there is no distinction between Muslims or non-Muslims in this respect; the verse refers to any person, which is remarkably important.
Murdering of an innocent person on purpose requires retaliation (qisas), namely killing the murderer. If the inheritors of the murdered person forgive the murderer, in this case, he needs to give blood money.
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