In Bereshis 4:8 we read: וַיֹּאמֶר קַיִן, אֶל-הֶבֶל אָחִיו; וַיְהִי בִּהְיוֹתָם בַּשָּׂדֶה, וַיָּקָם קַיִן אֶל-הֶבֶל אָחִיו וַיַּהַרְגֵהוּ
And Cain spoke unto Abel his brother. And it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him.
Why did Kayin do this? I recall learning that they argued over a who was able to marry a second twin sister. I'm not sure what the source of this is but it seems to me a rather empty reason for murder.