Srila Visvanatha Cakravarti Thakura points out that Krishna’s apparent punishment of the demons or unrighteous is in fact an act of mercy because the final result of this punishment is liberation. Srila Baladeva Vidyabhusana in his Govinda Bhasya commentary on Vedanta-sutra says that the demons killed by Krishna experience not only the death of their gross material body but the demise of their subtle body as well. The subtle body carries the soul from one gross body to another. The subtle body consisting of a state of mind is the basis of the gross body that one acquires in the next life. When the subtle body of the unrighteous person is destroyed by Krishna, the person’s attitude toward him immediately changes, for his opposition to Krishna was a product of his subtle body–his unrighteous disposition. On the demise of the subtle body, his hostility toward Krishna is transformed into love for Krishna. Thus at the moment of death he sees Krishna as the greatest object of affection. His liberation is not directly a result of being killed by Krishna but rather the result of his newfound love for him.
Excerpt from Sanga: Liberating the Demons.