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 קַיִן אֶל-הֶבֶל אָחִיו וַיַּהַרְגֵהוּ
"Kayin said to his brother Hevel. And when they were in the field, Kayin rose against his brother Hevel and killed him."
We understand that Kayin spoke to his brother, but there was no account of what he said to Hevel.
Chazal in Braishit Rabah, 22:16 quotes three different opinions as to what Kayin said to Hevel. In each case, there was an argument which led to Kayin killing Hevel. Whether they argued over the division of the earth's possessions (one receiving movable objects and the other acquiring land and property), or in whose property the Beit HaMikdash would be built or if they fought over their sister, these three opinions depict the genesis of fighting and war: People fight over land and property, religion and passion.
Yet, the Torah does not mention what the real reason was, because, perhaps, it is irrelevant. No matter what caused Kayin's violent outrage and murderous act, it can never justify such a horrific outcome - the killing of another.
Kayin's allowing envy or ego to overtake a valued life - a creation in the image of God was his failure. All too often, when family, friends and community members fight with one another, there is a risk of something we deeply value much more will be sacrificed for the sake of proving ourselves right or feeling heard.
Perhaps that explains Kayin’s punishment was “na va-nad tiheyeh ba-aretz” – he would forever be a wanderer, incapable of permanent settlement in one place. When a person insists on procuring what he or she thinks is coming to him or her no matter what the cost and protest even the smallest injustices, it will be difficult to feel grounded and stable. If we go to war for every negative remark or feeling that someone is taking what is rightly ours, becoming territorial, we will find ourselves in a state of “na va-nad tiheyeh ba-aretz,” of constant struggle for what we feel we need but do not have. Embedded in the early stories of humanity, is one of a person who needs to accept his reality even when it did not meet with his expectations.
If we would like to be content in our lives and experience stability, then we must recognize that life provides us with adversity and push back. Without that awareness, we will experience life through the struggle of "na va nad", chasing a reality of perfection in life which will never be.