"The fifth commandment says Thou shall not kill!"
It is a mistranslation. The commandment initially was: "You do not have to kill outside the law". So it is forbidden death, not death penalty, which is known in the Bible that was a common habit that time. However, the challenge is wrong translated: "Eye for eye, tooth for tooth". In reality it was not revenge but repair of iniquity. Perpetrator had to give something to the defendant, equivalent to what was stolen.