ובא האות והמופת, these false prophets came by their knowledge by means of their dealings with the necromancers, etc., people whom we are supposed to put to death.

'כי מנסה ה' וגו, the only reason G’d allowed such people to possess what sounds like supernatural knowledge, is to test our faith, to see if we will heed the prohibition against using such sources of information and of tolerating such people as part of our culture. By choosing not to resort to the knowledge of such people of the future, etc., we accumulate merits in G’d’s books (compare Deut. 18,10-13). Refusing to believe such false prophets is in itself a good deed. Believing them is equivalent to believing in idolatry.

יסיתך, every advice the end result of which leads to disaster is called הסתה; examples of such meanings of the word הסתה occur in Job 2,3 ותסיתני לבלעו חנם, You have incited me against him for no reason.” Or, Kings I 21,25 אשר הסתה אותו איזבל אשתו, “at the instigation of his wife Izzevel.” Or, Chronicles II 18,2 ויסיתהו לעלות אל רמות גלעד, “he incited him to march against Ramot Gilead’ (a campaign which failed and cost Achav’s life)

כי הרג תהרגנו, the word כי means אלא, but, i.e. but be sure to execute him.