What does Hashem Elokecha ask from you, but to fear Hashem Elokecha and to walk in His ways? (Devorim, Perek Yud, Pasuk Yud Gimmel)

This D'var Torah is based on a vort from the Kedushas Halevi.

The Chumash tell us that Zimri ben Salua took a woman from Midyan and was intimate with her in front of the Am. Pinchas was enraged by what he saw and took a sword and killed him. The Gemara tells us that Pinchas was Eliyahu Hanavi. The Ari HaKadosh explains that Pinchas was Kayin. Now this is a little harder to understand. That Pinchas - a Tzaddik who felt kinah for the Ribbono Shel Olam and acted upon it is Eliyahu, makes sense. But Kayin? A man who refused to bring the best of his crops as a karban? A man who murdered his brother and tried to hide it - this is the man who became Eliyahu? How can we explain this?

Let's look at Kayin. Kayin's aveira was r'zecha - murder. Kayin was his father's eldest, which made him a Kohen. When Hashem told him that the blood of his brother was screaming from the ground, Kayin answered "Hashomer Achee Anochi"? Am I my brother's keeper?

Now let us look at Pinchas and Eliyahu. Pinchas, a Kohen, took Kayin's sin of murder and turned it into a mitzvah. This was the first tikun. The second tikun will come at the time of the Geulah when Eliyahu will announce the coming of Moshiach. Kayin said "Hashomer Achee Anochi" - we usually read it as a quesion. But it wasn't - it was a statement - I am my brother's keeper. Pinchas took responsibility for his brother's actions and obliterated a blight on the Am. At a bris we have "Kiseh Eliyahu" - a special seat for Eliyahu. This is because at a bris, we cut a part of the body that is later buried in the ground - to make up for the blood of Hevel that called out for justice.

Everything happens in stages - a neshama could not go from Kayin to being Eliyahu Hanavi - first he became Pinchas and was metaken on the sin of murder - then he could become Eliyahu and be metaken on the sin of spilling blood on the ground.