Ramban

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 them so that they would not have to spill his blood with their own hands. Thus did the Rabbis say in Bereshith Rabbah: “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.”