informer

An informant, especially one who informs against others for compensation.

One who informs or animates.

One who imparts intelligence or gives information; an informant.

In law, one who communicates to a magistrate a knowledge of a violation of law; a person who lays an information against or prosecutes in the courts one who offends against the law or any penal statute.

Hence One who makes a business of informing against others; a mischief-maker.