Why did Cain kill Abel? Did he observe his younger brother's sacrifices and think, "I, too, am a firstborn - perhaps he means to sacrifice me as he sacrificed those sheep"? And so, projecting his own violent impulses on his brother?
We know what happened next. This is the original crime, man against man. And it's interesting that the motive was not for some earthly gain - wealth or power or the love of a woman - but jealousy over Abel's favor with the Creator.
It is the easiest human failing to look at someone else who is doing well, someone who succeeds and excels, and instead of asking "How can I do better?", to ask instead, "How can I bring him down?"