Bereishit chapter 4 verse 8 says:
"Kayin said to his brother Hevel...and it happened that when they were in the field that Kayin rose up against his brother Hevel and killed him."There seems to be some missing words. What did Kayin say to Hevel before he killed him?
There are many different answers given by the commentaries. Rashi refers us to a midrash in Berishit Rabbah. The midrash says that although the primary cause of the fight was Kayin's jelousy, Kayin and Hevel had a separate argument that sparked the fuse that actually persipitated the murder.
There are three opinions as to what that argument was.
Version 1: Kayin said to Hevel, hey, the land belongs to me, get off my land. hevel then said, fine, but I provided you with clothes so strip and give them back to me. according to this version the fight was over property.
Version 2: Kayin and Hevel decided that they should build a Temple to Hashem. Kayin said wanted the Temple on his property while Hevel wanted it on his. According to this version it was a religious conflict.
Version 3: Kayin and Hevel were each born with a twin sister but Hevel and his sister were a triplet set. Kayin and Hevel each married one of their sisters and then fought over who should get the second wife. Hevel argued that he should since it was his sister, while Kayin argued he should since he was older. According to this version the fight was over a woman.
I read in Rabbi Jonathan Sach's book "The Dignity of Difference" that the midrash is setting up the fight of Kayin and Hevel as a paradigm for all future conflicts to draw lessons from. The three versions are meant to capture the various different reasons why individuals fight or why nations go to war. It is usually a conflict over money, religion, or a woman (like Helen of Troy, the face that launched a thousand ships).