Sahih International: Because of that, We decreed upon the Children of Israel that whoever kills a soul unless for a soul or for corruption [done] in the land – it is as if he had slain mankind entirely. And whoever saves one – it is as if he had saved mankind entirely. And our messengers had certainly come to them with clear proofs. Then indeed many of them, [even] after that, throughout the land, were transgressors.
The following excerpt is taken from “Anwar ul Bayan” pg. 68:
“Qurtubi (v. 6 p. 146) reports many interpretations for this verse. Sayyidina Mujahid (may Allah be pleased with him) states that whoever murders another will suffer the punishment that Allah shall be angry with him, will curse him and subject him to the great torment of Hell. This has been discussed in verse 93 of Surah Nisa. Since the person who kills the entire mankind will also suffer the same combination of punishments, killing one will be like killing all mankind.
According to others, the sin for killing one person is the same as killing all people. This is closest to the words of the verse. In a hadith the Holy Prophet (peace be upon him) has said, ‘Whenever a murder is committed, the sin will be shared by the first son of Sayyidina Adam (peace be upon him) because he was the first to begin the practice of murder.” [Bukhari and Muslim]”
As well on the same page of the book:
“Allah then speaks on the virtue of saving lives. He says, “Whoever preserves a life, it is as if he has preserved the lives of the entire mankind. Undoubtedly Our messengers have come to them with clear signs. Thereafter many of them transgressed on the earth.”
Lesson: The above hadith teaches us that the person who initiated any evil deed shall receive the sin of people doing the same without their sins being decreased. In the same way, the person who initiates a good deed will receive the reward of all those emulating him or acting upon his preaching without any of their rewards being diminished in the least. [Mishkat p.33]”