ויחרד יצחק חרדה גדלה עד מאד ויאמר מי אפוא הוא הצד ציד ויבא לי ואכל מכל (כז-לג)
The Torah tells us that when Yitzchok realized that Yaakov had taken the berachos from Esav, “Vayecherad charada gedola ad meod,” he trembled a great and awesome trembling. Rashi says that he saw Gehinom open up beneath him. What was it that caused Yitzchok to tremble in this extreme way? The Medrash Tanchuma tells us that there was another time in Yitzchok’s life that he trembled. The Medrash explains that when Avraham lifted the knife over Yitzchok’s head at the Akeida, he trembled. Why did he tremble? Because he was afraid that he might move and invalidate himself as a korban. He was afraid that he would not be a perfectly fitting sacrifice for Hashem. The Medrash asks: Which trembling was greater? And the answer is that it was the trembling at the time of the giving of the brachos, as the posuk says, “Charada gedola AD MEOD.”
Explains the Shevet Sofer, Yitzchok Avinu was lying on the altar, and he was scared. He was afraid that his performance bein adam l’Makom might not be up to par, and so the Medrash tells us that he trembled - “Vayecherad.” However, in Parshas Toldos, when Esav came to Yitzchok to get his beracha, Yitzchok realized that he had already given it to Yaakav. He understood that now he might be causing pain and harm bein adam l’chaveiro - to Esav - even though the berachos were his to give to whomever he wanted. Esav was his own son and he gave the berachos to Yaakov unintentionally. It’s not the end of the world. Still, he trembled in a way that he had never trembled before. “Vayecherad charada gedola AD MEOD.” This would be a mistake that would have terrible ramifications on his olam haze and his olam haba. Chazal tell us “Hizaharu B’chvod Chavreichem” - Be careful with the honor of your friends. Playing with someone’s feelings is much more dangerous than playing with fire. Fire only burns in this world, but causing someone pain burns in this world and in the next.
The Chazon Ish zt”l put it quite succinctly. “The kuntz (the trick) of life is to get through this world without hurting."