If any one slew a person, it would be as if he slew the whole people.
Let's look more closely at this Verse, in its not abbreviated version:
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 any one saved a life, it would be as if he saved the life of the whole people. Then although there came to them Our messengers with clear signs, yet, even after that, many of them continued to commit excesses in the land.
Its meaning looks quite different now, doesn't it ?
- The verse is an order concerning first of all Jews. It in fact condemns the Jews because they don't followed Allah's orders. It is clearly one of the antisemitic verses.
- The verse condemns the killing, unless a victim has committed a murder or spread a "mischief in the land". In other words a person who opposes the rule of Islam can be killed.
The punishment of those who wage war against Allah and His Messenger, and strive with might and main for mischief through the land is: execution, or crucifixion, or the cutting off of hands and feet from opposite sides, or exile from the land: that is their disgrace in this world, and a heavy punishment is theirs in the Hereafter.
Which doesn't look peaceful at all and puts everything in a completely different perspective.
Islam has borrowed (and turned upside-down) this verse from Talmud, where it has its original meaning.
You can find it in Mishnah Sanhedrin 4:9:
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.