The 292nd prohibition is that we are forbidden from killing someone who has already done1 something punishable by death before he is brought to High Court. Rather, we must bring him to High Court and present the evidence against him. We must be only witnesses [not judges], and they will determine his guilt.
The source of this prohibition is G‑d's statement2 (exalted be He), "The murderer shall not die until he stands before the court in judgment."
In the words of the Mechilta,3 "One might think that [the witnesses] may kill the person after he has committed murder or adultery. The Torah therefore says, 'The murderer shall not die until he stands ...' " Even if the Great Sanhedrin saw him commit the murder, they may only be witnesses and testify before another court [rather than themselves act as judges]; and that court will execute him.
The Mechilta also says, "In a case where a court saw a person commit murder, you might think that they may execute him even before he is judged. The Torah therefore says, 'The murderer shall not die until he stands before the court in judgment.' "
