Assassinating Viscount Tuli Yang so publicly was already the lesser of the crimes.
Greater crimes included subverting the tradition of Yin and Yang, attempting to elevate a woman to power. Then there was the use of Sky-shattering Thunder, damaging Sky Harmony, defying the Dragon Power, and so on and so forth!
In short, in this sacrificial scripture, Prince Zhi Li listed thirteen grave sins committed by Suo Lun.
Every single one of these sins was punishable by death, with no chance for forgiveness.
This time, it wasn't Prince Zhi Li who was to annihilate Suo Lun, but heaven itself. Prince Zhi Li was merely the instrument of divine retribution.
Hence, the profound proposition: Suo Lun must be punished by heaven.