Albert Barnes on Acts 28:6

When he should have swollen. When they expected he would have swollen from the bite of the viper. The poison of the viper is rapid; and they expected that he would die soon. The word rendered "swollen"--πιμπρασθαι--means, properly, to burn, to be inflamed, and then to be swollen from inflammation. This was what they expected here, that the poison would produce a violent inflammation.

Or fallen down dead suddenly. As is sometimes the case from the bite of the serpent, when a vital part is affected.

They changed their minds. They saw he was uninjured, and miraculously preserved; and they supposed that none but a god could be thus kept from death.

That he was a god. That the Maltese were idolaters there can be no doubt. But what gods they worshipped is unknown, and conjecture would be useless. It was natural that they should attribute such a preservation to the presence of a divinity. A similar instance occurred at Lystra. Acts 14:11.

(+) "Howbeit" "However" (++) "looked" "expected" (g) "that he was a god" Acts 14:11