After the sword went through the camp, the Levites stood with blood on their hands. They had killed brothers, neighbors, friends. And Moses turned to them with a startling instruction.
Targum Pseudo-Jonathan, the Aramaic paraphrase of the Torah, renders the verse this way. "Offer your oblation for the shedding of the blood that is upon your hands, and make atonement for yourselves before the Lord, because you have smitten a man his son or his brother, and that you may bring a blessing upon you this day" (Exodus 32:29).
Read it again. The very act that consecrated the Levites to priesthood also required kapparah, atonement. They were blessed for the killing. They also had to seek forgiveness for it.
This is the Targum at its most theologically sophisticated. A righteous deed performed with violence remains, in its effect on the soul, something that must be cleansed. The Levites had done what had to be done. They had also spilled blood that was, in another sense, family blood. The blessing and the atonement were not contradictions. They were the truth of what serving God sometimes demands and always costs.
This is why the tribe became priests. Not because they were most eager for violence, but because they had learned its weight.
Takeaway: A necessary action still leaves a mark on the soul. The righteous do not celebrate the sword. They atone for having to draw it.