2 Samuel 6:7

And the anger of the Lord was kindled against Uzzah
And which was manifest by smiting him:

and God smote him there for [his] error;
committed at this time, which was complicated; as that the ark was put upon a cart, to which he might be the chief adviser, as Procopius Gazaeus notes, when it should have been carried on the shoulders of the Levites; and that be touched it with his hand, which none but priests might do, supposing him to be a Levite, which it is not clear he was, however not a Kohathite; and he took hold of it in order to carry it in his arms, which even Levites, and those Kohathites, might not do, but with staves put into it, which only they were to hold; and besides, as Abarbinel observes, he showed little faith in the power and providence of God, as if he could not take care of the ark without him:

and there he died by the ark of God;
directly, upon the spot, by the side of it; whether he was struck by lightning, or in what way, cannot be said; however, he died by the immediate hand of God, in token of his displeasure: and this shows that it is dangerous in matters of worship to act contrary to the command of God, even in things that may seem small and trivial; and though what may be done may be done with a good intention, as this was, yet that will not excuse the sin; nor are those who are the most forward and zealous in religious matters exempted from marks of God's displeasure when they go wrong.