Rashi: Perhaps you normally give a man's wife to another man.

Radak: Had you done so when they stole my wife - to burn her and her father's house - it would be fine. Now that you were not concerned for this until I avenged the treachery done to me, I will take vengeance against you one more time, and then I will refrain.

Malbim: I thought that monetary punishment suffices (to burn your fields). Now that you punished mortally (burned her and her father), also I will punish mortally, like your custom!