Sforno on Genesis 26:35/ספורנו על בראשית כ״ו:ל״ה

1ותהיין מורת רוח, they became like a razor and a knife, shortening the lives of both Yitzchok and Rivkah. The expression מורת is the same as in Judges 13,5 where the angel told the wife of Manoach that the as yet unborn Shimshon must be a Nazirite and ומורה לא יעלה על על ראשו, no “hair-trimming tool must touch his head.” Even now Yitzchok did not realise the extent of his son Esau’s wickedness. He did not protest his behaviour with a single word. This led to his ultimate error in setting out to bestow the blessing on his son Esau. This, in turn, [i.e. Esau first having been promised a blessing and then not getting it Ed.], led to Esau getting the blessing or advice that he would live (be successful) by his sword. Because of this blessing he hated his brother to the extent that Yaakov had to flee for his very life to another country in order to get away from his murderous intentions.