Allah says, because the son of Adam killed his brother in transgression and aggression,

﴿كَتَبْنَا عَلَى بَنِى إِسْرَءِيلَ﴾

(We ordained for the Children of Israel...) meaning, We legislated for them and informed them,

﴿أَنَّهُ مَن قَتَلَ نَفْساً بِغَيْرِ نَفْسٍ أَوْ فَسَادٍ فِى الاٌّرْضِ فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعاً وَمَنْ أَحْيَـهَا فَكَأَنَّمَا أَحْيَا النَّاسَ جَمِيعاً﴾

(that if anyone killed a person not in retaliation of murder, or (and) to spread mischief in the land - it would be as if he killed all mankind, and if anyone saved a life, it would be as if he saved the life of all mankind.) The Ayah states, whoever kills a soul without justification -- such as in retaliation for murder or for causing mischief on earth -- will be as if he has killed all mankind, because there is no difference between one life and another.

﴿وَمَنْ أَحْيَـهَا﴾

(and if anyone saved a life...) by preventing its blood from being shed and believing in its sanctity, then all people will have been saved from him, so,

﴿فَكَأَنَّمَا أَحْيَا النَّاسَ جَمِيعاً﴾

(it would be as if he saved the life of all mankind.) Al-A`mash and others said that Abu Salih said that Abu Hurayrah said, "I entered on `Uthman when he was under siege in his house and said, `I came to give you my support. Now, it is good to fight (defending you) O Leader of the Faithful!' He said, `O Abu Hurayrah! Does it please you that you kill all people, including me' I said, `No.' He said, `If you kill one man, it is as if you killed all people. Therefore, go back with my permission for you to leave. May you receive your reward and be saved from burden.' So I went back and did not fight.''' `Ali bin Abi Talhah reported that Ibn `Abbas said, "It is as Allah has stated,

﴿مَن قَتَلَ نَفْساً بِغَيْرِ نَفْسٍ أَوْ فَسَادٍ فِى الاٌّرْضِ فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعاً وَمَنْ أَحْيَـهَا فَكَأَنَّمَا أَحْيَا النَّاسَ جَمِيعاً﴾

(if anyone killed a person not in retaliation of murder, or (and) to spread mischief in the land - it would be as if he killed all mankind, and if anyone saved a life, it would be as if he saved the life of all mankind.) Saving life in this case occurs by not killing a soul that Allah has forbidden. So this is the meaning of saving the life of all mankind, for whoever forbids killing a soul without justification, the lives of all people will be saved from him.'' Similar was said by Mujahid;

﴿وَمَنْ أَحْيَـهَا﴾

(And if anyone saved a life...) means, he refrains from killing a soul. Al-`Awfi reported that Ibn `Abbas said that Allah's statement,

﴿فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعاً﴾

(it would be as if he killed all mankind. .) means, "Whoever kills one soul that Allah has forbidden killing, is just like he who kills all mankind.'' Sa`id bin Jubayr said, "He who allows himself to shed the blood of a Muslim, is like he who allows shedding the blood of all people. He who forbids shedding the blood of one Muslim, is like he who forbids shedding the blood of all people.'' In addition, Ibn Jurayj said that Al-A`raj said that Mujahid commented on the Ayah,

﴿فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعاً﴾

(it would be as if he killed all mankind,) "He who kills a believing soul intentionally, Allah makes the Fire of Hell his abode, He will become angry with him, and curse him, and has prepared a tremendous punishment for him, equal to if he had killed all people, his punishment will still be the same.'' Ibn Jurayj said that Mujahid said that the Ayah,

﴿وَمَنْ أَحْيَـهَا فَكَأَنَّمَا أَحْيَا النَّاسَ جَمِيعاً﴾

(and if anyone saved a life, it would be as if he saved the life of all mankind.) means, "He who does not kill anyone, then the lives of people are safe from him.''