“And the murderer will not die until he stands before the assembly for judgment!” (Bamidbar 35:12)
It is a negative commandment that one may not kill a sinner, even if we see him committing the crime which deserves the death penalty, before we bring him to the beis din (Jewish Court of Law). We are obligated to bring him to the beis din and to bring witnesses in front of them, and they will judge him if he is guilty, as it says “And the murderer will not die until he stands before the assembly for judgment!”. This law is true even if the grand beis din saw him killing someone that they can all act as witnesses and bring him to another beis din to be judged.
Amongst the roots of this commandment are that life and death judgments is a very difficult thing and needs especially good comprehensive attention to all the details. The court has been commanded to save the accused in any way possible. This does not mean that they should, G-d forbid, bend the law, but they must try to find a way to exonerate him, and if they find a way, they must proclaim his innocence, and if not, he must be put to death. It follows that we have been warned that the case must at least come before the beis din, and the witnesses themselves may not judge him, for maybe by them seeing the story, they will be unable to find a merit for him, for their hearts will be aroused to find him guilty.
This commandment applies to men and to women at all time, as we are commanded not to kill any sinner even if we have seen him commit a sin for which he would receive capital punishment in the beis din. When the temple is standing we are obligated to bring him before the beis din so that they may judge him.