So I'll admit I feel a bit of sympathy for Kayin. Who here hasn't wanted to smack their bratty kid sibling around a little bit? Maybe you were tired of being followed around, or asked annoying, incessant questions. Maybe you were fighting for parental attention, or over a parental inheritance. Maybe you just knew it would be so much easier to break them, bruise them, bloody them -- but you didn't. Most of us get into fights with family. But most of us also know the consequences of physical violence, and are therefore unwilling to use it.
It struck me how innocent (if petulant and petty...sorta like a kid can be) Kayin comes across in some of this section of agadot about him and his unfortunate brother Hevel.
"Kayin said [to God]: Ruler of the World! I've never seen or known of death in my whole life! How was I to know that if I hit [Hevel] with a rock, he would die?"
"How did [Kayin] kill [Hevel]? He grabbed a rock and would not stop injuring and wounding him with it. He broke his arms and his legs, not knowing the exit through which life leaves the body, until he reached [Hevel's] neck and he died."
I remember being struck by Steg of http://boroparkpyro.blogspot.com 's analysis and dramatization of the incident, found here. He points out that if you keep an close eye on the grammar in this section of the Torah, the evidence points to Kayin having been born inside the Garden of Eden. Unlike Hevel, who had been born on the outside from parents who had ingested the knowledge of good, evil, and death ("for on the day you eat of it you will surely die"), it may be that Kayin could not "even understand what death was, until he created it with his own hands".
And Kayin wasn't the only one for whom death was new and bewildering. I remember how lost and confused I felt when I received the phone call that my father (ע"ה) had died, and I live in a universe where thousands of people pass on every second. But I needed know what to do, and had no idea. Thankfully I had an unofficial rabbi and friends to guide me through the subsequent steps and responsibilities. But who did the First Man and First Woman have to teach them?
"Adam and [Chavah] arrived and sat next to [Hevel's] body. They cried and mourned but did not know what to do for Hevel. A raven whose comrade had died said, 'I will teach this Adam what to do.' He took his comrade, dug in the dirt in front of [Adam & Chavah], and buried him. Adam said, 'We will act like this raven.' He immediately took Hevel's body and buried it in the ground."