HaRav Avigdor Miller zts"lRevered rabbi, author and gifted lecturer in USA who studied at YU and Slobodka, mashgiach ruchani (spiritual advisor) of Rabbi Chaim Berlin Yeshiva and later head of Beit YIsrael Yeshiva. Aug. 28, 1908 - Apr. 20, 2001. Rabbi Avigdor Miller Sent by Simchas Hachaim Foundation, based on the works of Rabbi Avigdor Miller (1908-2001). Discover more at SimchasHachaim.com.
The Torah does not reveal what was said, and therefore we ask why does the Torah mention that "Cain said to this brother?" But we learn here that the tragedy would not have occurred if not for the words that were exchanged. Had nothing been said, or had Hevel not replied, the crisis would have passed.
But when Cain spoke and Hevel replied, and the passion was inflamed, the catastrophe happened.
Here the Torah teaches the urgency of retraining from words at a time when emotions are inflamed: "And if you thought (evil), a hand upon the mouth!" (Mishle 30:32).
The tragedy resulted because "Cain said to (quarrelled with) his brother," instead of putting his hand upon his mouth. (From In the Begining)