"and you shall not shield him": If you know something against him, you may not remain silent.
Whence is it derived that if he leaves beth-din incriminated, he is not to be returned for acquittal? From (Devarim, Ibid. 10) "But kill (shall you kill him.")
And whence is it derived that if he leaves beth-din acquitted, he is to be returned for incrimination? From "shall you kill him."
"Your hand shall be against him first to put him to death": It is a mitzvah for the one incited to put him to death, and, afterwards, (if he did not die under his hand), by all men, it being written "and the hand of all the people afterwards."