Rashi on Genesis 37:18/רש"י על בראשית ל״ז:י״ח

1ויתנכלו AND THEY CONSPIRED — The Hithpael form denotes that they became filled with plots and craft.

2אֹתוֹ is here the same as אִתּוֹ which means “with him” — meaning אליו: they became filled with plots and craft directed towards him (אליו).