Commentary for Genesis 37:18

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

AND THEY CONSPIRED AGAINST HIM TO SLAY HIM. They thought to kill him with their subtle intrigues by which they had conspired against him before he drew near to them⁶⁵According to Tur this refers to attempting to kill him with arrows. so that they would not have to spill his blood with their own hands. Thus did the Rabbis say in Bereshith Rabbah:⁶⁶84:13. “Let us set the dogs against him.” And perhaps they did so but did not succeed. Now, when they saw that he was approaching them and they could not kill him with their intrigues, they said to each other, “Behold, he has come to us, so let us kill him ourselves.”

ויתנכלו אותו, they were full of devious and nefarious thoughts. The word occurs in this sense in Maleachi 1,24: וארור נוכל, “cursed the person who plots deviously.”