הָפַ֣ךְ לִ֭בָּם לִשְׂנֹ֣א עַמּ֑וֹ לְ֝הִתְנַכֵּ֗ל בַּעֲבָדָֽיו׃
Leur cœur changea jusqu’à prendre son peuple en haine, et ourdir des machinations contre ses serviteurs.
Radak
ויתנכלו אותו, the word אותו here means the same as the preposition בו, “against him.” We find other instances where this preposition is used in connection with this root, as in Psalms 105,25 להתנכל בעבדיו, “to plot against His servants.” The root also occurs in connection with the preposition ל, as in Numbers 25,18 אשר נכלו לכם, “which they conspired against you.” The word basically refers to scheming against someone in one’s heart.