Whoever destroys a soul, it is considered as if he destroyed an entire world. And whoever saves a life, it is considered as if he saved an entire world.
– Mishnah Sanhedrin 4:5, Talmud Sanhedrin 37a
We ordained for the Children of Israel that if any one 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 anyone saved a life, it would be as if he saved the life of the whole people
– Quran 5:32
There is no equivalency between the slaughter of one person and the genocide of mankind, in either Judaism or Islam. What the Quran and Jewish Oral Law are teaching that if someone takes a life they kill more than one person as they deny all of the future generations that could have been.
This is reference to the story of Cain and Abel is intended to force the reader to imagine how many people could have descended from Abel (the son of Adam) had he survived.