Opposition to ancient Near East custom
It was a widespread custom, both in Eastern and Western countries, and accepted also in Israelite usage (i Kings ii 28 ff.; see on this point above, p. 261), that whoever entered a sacred place was saved from all punishment, even if he had killed a person willfully. The Torah abolishes this practice in the case of deliberate murder; the sanctity of the temple cannot override the sanctity of human life.
That puts this Exodus law in opposition to the "ordeal" (which was a commonly-used attempt to force judicial knowledge from God (or the gods)). The willful violation of a holy place would no longer signal anything about God's judicial assessment.
Just another example of how Biblical law opposed the law systems of the surrounding nations.
- Cassuto, A Commentary on the Book of Exodus, 270