Deuteronomy 13:9

But thou shalt surely kill him; thine hand shall be first upon him to put him to death, and afterwards the hand of all the people.

Thou shalt ... kill him - not hastily, or in a private manner, but after trial and conviction; and his relative, as informer, was to cast the first stone (see the notes at Deuteronomy 17:7; Acts 7:58). It is manifest that what was done in secret could not be legally proved by a single informer; and hence, Jewish writers say that spies were set in some private part of the house to hear the conversation and watch the conduct of a person suspected of idolatrous tendencies.

The reason why idolatry was punished with death was this, that the Hebrew government was a theocracy; and the unity of the Godhead being a fundamental law of the state, idolatry was treason; which in every kingdom is regarded as the highest crime, and visited with capital punishment. It was therefore placed (as in Deuteronomy 13:11) under the ban of absolute, unqualified prohibition.