Charles Ellicott Commentary 2 Samuel 6:7

"And the anger of Jehovah was kindled against Uzzah; and God smote him there for his error; and there he died by the ark of God." — 2 Samuel 6:7 (ASV)

For his error. —The original is here very obscure: 1 Chronicles 13:10 has because he put his hand to the ark. (Compare to 1 Samuel 6:19.) Especial sacredness was by the law attached to the ark, and it was strictly commanded that when it was to be moved, it should be first covered by the priests and then borne by the Levites by means of its staves; but until it was covered, the Levites were not to look upon it and were not to touch it, on pain of death (Numbers 4:5; Numbers 4:15; Numbers 4:19–20). Uzzah was probably a Levite, or, at any rate, had been so long in the house with the ark that he ought to have made himself familiar with the law in regard to it.

What may seem, at first thought, an exceedingly severe penalty for a well-meaning, though unlawful act, is seen on reflection to have been a very necessary manifestation of the Divine displeasure; for this act involved not only a violation of the letter of the law (of which David also was guilty), but a lack of reverence for the majesty of God as symbolized by the ark, and showed a disposition to profane familiarity with sacred things. “Uzzah was a type of all who, with good intentions, humanly speaking, yet with unsanctified minds, interfere in the affairs of the kingdom of God, from the notion that they are in danger, and with the hope of saving them” (O. von Gerlach). Judgments of this kind were, however, temporal, and give in themselves no indication of the treatment of the offender beyond the grave.