Mishnah Sanhedrin 4:5

(5) How do we get the witnesses in a capital case to understand the seriousness of their job?

We bring them in (to the court's chambers) and impress upon them: "Perhaps what you say is your own assessment of what happened (rather than what you actually saw), or from rumors, or your witnessing an actual witness testify, or your reporting what a trustworthy said. Or perhaps you were unaware that by the end we'd interrogate you, with examination and inquiry.

Know that capital cases are not like monetary ones. In monetary cases, (a false witness) can give money and achieve atonement. But in capital cases, the blood of (the victim) and all his future offspring hang upon you until the end of time. As we find with Cain, who killed his brother, "The bloods of your brother scream out!" (Genesis 4:10) - the verse does not say blood of your brother, but bloods of your brother, because it was his blood and also the blood of his future offspring (screaming out)! [Another explanation of the verse: for his blood was splattered over the trees and rocks (i.e. there was more than one pool of blood).]

(The judges' speech continues:) "It was for this reason that man was first created as one person, to teach you that anyone who destroys a life is considered by Scripture to have destroyed an entire world; any who saves a life is as if he saved an entire world." And also, to promote peace among the creations, that no man would say to his friend, "My ancestors are greater than yours." And also, so that heretics will not say, "there are many rulers up in Heaven." And also, to express the grandeur of The Holy One [blessed be He]: For a man strikes many coins from the same die, and all the coins are alike. But the King, the King of Kings, The Holy One [blessed be He] strikes every man from the die of the First Man, and yet no man is quite like his friend.] Therefore, each and everyone must say, 'for me the world was created.'"(The judges' speech continues:)"Maybe you will now say, 'What do we need this, and all this anxiety for (let's not come forward even with true testimony)?' But Scripture has already spoken: "If he be a witness - having seen or known - if he does not express it, he shall bear his sin." (Lev. 5:1) Maybe you will now say, 'What do we need this, to be responsible for another man's death?' But Scripture has already spoken: "When the wicked are destroyed there is rejoicing." (Proverbs. 11:10)"