Aside from the tragic nature of the event of Kayin killing his brother Hevel, there is a great deal of ambiguity as to what took place before the murder.  
The Torah says Perek 4, pasuk Chet:  

וַיֹּאמֶר קַיִן, אֶל-הֶבֶל אָחִיו; וַיְהִי בִּהְיוֹתָם בַּשָּׂדֶה, וַיָּקָם...

Shabbat Chol haMoed Sukkot: The Haftarah read on Shabbat Chol HaMoed Sukkot

The Haftara read on the Shabbat of Chol HaMoed Sukkot comes from Sefer Yechezkel, Chapter 38.  This is the prophecy of Gog U'Magog.

Gog is a ruler over Magog who attacks the Jewish people...