whoever saves a single life

link
"In one famous instance, the legal code turned almost poetiic, as Rabbi Judah [the Prince] cited the lengthly warning in the rabbinic judges delivered to witnesses testifying in capital cases:

'How are witnesses inspired with awe in capital cases?' the Mishna begins. 'They are brought in and admonished as follows:  In case you may want to offer testimony that is only conjecture or hearsay or secondhand evidence, even from a person you consider trustworthy; or in the event you do not know that we shall test you by cross-examination and inquiry, then know that capital cases are not like monetary cases.  In monetary cases, a man can make monetary restitution and be forgiven, but in capital cases both the blood of the man put to death and the blood of his (potential) descendants are on the witness's head until the end of time.  For thus we find in the case Cain, who killed his brother, that it is written:  "The bloods of your brother cry unto Me "(Gen. 4:10)--that is, his blood and the blood of his potential descendants....  Therefore was the first man, Adam, created alone, to teach us that whoever destroys a single like, the Bible considers it as if he destroyed an entire world.  And whoever saves a single life, the Bible considers it as if he saved an entire world.....'

Telushkin, Jewish Literacy, New York, 1991, pp. 151-52.