As stated in Bhagavad-gita (4.8), the Lord descends as an incarnation to deliver the devotees and kill the miscreants. The Lord of the universe and of everything would appear to kill the sons of Diti because of their offending the devotees of the Lord. There are many agents of the Lord, such as Indra, Candra, Varuna, goddess Durga, and Kali, who can chastise any formidable miscreants in the world. The example of mountains being smashed by a thunderbolt is very appropriate. The mountain is considered the most strongly built body within the universe, yet it can be easily smashed by the arrangement of the Supreme Lord. The Supreme Personality of Godhead does not need to descend in order to kill any strongly built body; He comes down just for the sake of His devotees. Everyone is subject to the miseries offered by material nature, but because the activities of miscreants, such as killing innocent people and animals or torturing women, are harmful to everyone and are therefore a source of pain for the devotees, the Lord comes down. He descends only to give relief to His ardent devotees. The killing of the miscreant by the Lord is also the mercy of the Lord towards the miscreant, although apparently the Lord takes the side of the devotee. Since the Lord is absolute, there is no difference between His activities of killing the miscreants and favoring the devotees.