John Calvin Commentary Genesis 34:27

"The sons of Jacob came upon the slain, and plundered the city, because they had defiled their sister." — Genesis 34:27 (ASV)

The sons of Jacob came. Moses shows that, not content with simple revenge, they hasten together to the plunder. Regarding the words, they are said to have come upon the slain, either because they made their way over the slaughtered bodies, or because, in addition to the slaughter, they rushed to the plunder.

In whichever way it is taken, Moses teaches that, not satisfied with their former wickedness, they made this addition to it. Granted, they were blinded with anger in shedding blood; yet by what right do they sack the city? This certainly cannot be ascribed to anger. But these are the ordinary fruits of human intemperance: whoever gives himself free rein in perpetrating one wickedness soon breaks out into another.

Thus the sons of Jacob, from being murderers, also become robbers, and the guilt of avarice is added to that of cruelty. We should therefore be more diligent in our efforts to bridle our desires, lest they mutually fan each other, so that eventually, by their combined action, a dreadful conflagration arises. Especially, we must beware of using force of arms, which brings with it many perverse and brutal assaults.

Moses says that the sons of Jacob did this because the Shechemites had defiled their sister; but the whole city was not guilty. Moses, however, only describes the mindset of the authors of the slaughter: for although they wish to appear as just avengers of the injury, they disregard what was lawful for them to do, make no attempt to control their depraved affections, and consequently set no bounds to their wickedness.

If anyone prefers taking the expression in a higher sense, it may be referred to the judgment of God, by which the whole city was involved in guilt because no one had opposed the lust of the prince. Perhaps many had consented to it, not being very much concerned about the unjust dishonor done to their guests. However, the former sense is the one I favor.