The venomous beast - The English word “beast” we usually apply to an animal of larger size than a viper. But the original θηρίον thērion is applicable to animals of any kind, and was especially applied by Greek writers to serpents. See Schleusner.
No doubt - The fact that the viper had fastened on him; and that, as they supposed, he must now certainly die, was the proof from which they inferred his guilt.
Is a murderer - Why they thought he was a murderer rather than guilty of some other crime is not known. It might have been:
(1) Because they inferred that he must have been guilty of some very atrocious crime, and as murder was the highest crime that man could commit, they inferred that he had been guilty of this. Or,
(2) More probably, they had an opinion that when divine vengeance overtook a man, he would be punished in a manner similar to the offence; and as murder is committed usually with the hand, and as the viper had fastened on the hand of Paul, they inferred that he had been guilty of taking life. It was supposed among the ancients that persons were often punished by divine vengeance in that part of the body which had been the instrument of the sin.
Whom, though he hath escaped the sea - They supposed that vengeance and justice would still follow the guilty; that, though he might escape one form of punishment, yet he would be exposed to another. And this, to a certain extent, is true. These barbarians reasoned from great original principles, written on the hearts of all people by nature, that there is a God of justice, and that the guilty will be punished. They reasoned incorrectly, as many do, only because that they supposed that every calamity is a judgment for some particular sin. People often draw this conclusion, and suppose that suffering is to be traced to some particular crime, and to be regarded as a direct judgment from heaven. See the notes on John 9:1-3. The general proposition that all sin will be punished at some time is true, but we are not qualified to affirm of particular calamities always that they are direct judgments for sin. In some cases we may. In the case of the drunkard, the gambler, and the profligate, we cannot doubt that the loss of property, health, and reputation is the direct result of specific crime. In the ordinary calamities of life, however, it requires a more profound acquaintance with the principles of divine government than we possess to affirm of each instance of suffering that it is a particular judgment for some crime.
Yet vengeance - ἡ δίκη hē dikē. “Justice” was represented by the pagan as a goddess, the daughter of Jupiter, whose office it was to take vengeance, or to inflict punishment for crimes.
Suffereth not to live - They regarded him as already a dead man. They supposed the effect of the bite of the viper would be so certainly fatal that they might speak of him as already, in effect, dead (Beza).