"There are two classes of human beings, namely the envious and the obedient. Since the Supreme Lord is one and the father of all living beings, the envious living beings are also His sons, but they are known as the asuras. But the living beings who are obedient to the supreme father are called devatas, or demigods, because they are not contaminated by the material conception of life. Not only are the asuras envious of the Lord in even denying the existence of the Lord, but they are also envious of all other living beings. The predominance of asuras in the world is occasionally rectified by the Lord when He eliminates them from the world and establishes a rule of devatas like the Pandavas. His designation as kala in disguise is significant. He is not at all dangerous, but He is the transcendental form of eternity, knowledge and bliss. For the devotees His factual form is disclosed, and for the nondevotees He appears like kala-rupa, which is causal form. This causal form of the Lord is not at all pleasing to the asuras, and therefore they think of the Lord as formless in order to feel secure that they will not be vanquished by the Lord."
"The asuras want to enjoy a life of sense gratification, even at the cost of others' happiness. In order to fulfill this ambition, the asuras, especially atheistic kings or state executive heads, try to equip themselves with all kinds of deadly weapons to bring about a war in a peaceful society. They have no ambition other than personal aggrandizement, and thus mother earth feels overburdened by such undue increases of military strength. By increase of the asuric population, those who follow the principles of religion become unhappy, especially the devotees, or devas. In such a situation, the Personality of Godhead incarnates to vanquish the unwanted asuras and to reestablish the true principles of religion. This was the mission of Lord Sri Krsna, and He fulfilled it."
"It is known that God is all-merciful, but here we find that God is never merciful to the demoniac. It is clearly stated that the demoniac people, life after life, are put into the wombs of similar demons, and, not achieving the mercy of the Supreme Lord, they go down and down, so that at last they achieve bodies like those of cats, dogs and hogs. It is clearly stated that such demons have practically no chance of receiving the mercy of God at any stage of later life. In the Vedas also it is stated that such persons gradually sink to become dogs and hogs. It may be then argued in this connection that God should not be advertised as all-merciful if He is not merciful to such demons. In answer to this question, in the Vedanta-sutra we find that the Supreme Lord has no hatred for anyone. The placing of the asuras, the demons, in the lowest status of life is simply another feature of His mercy. Sometimes the asuras are killed by the Supreme Lord, but this killing is also good for them, for in Vedic literature we find that anyone who is killed by the Supreme Lord becomes liberated. There are instances in history of many asuras--Ravana, Kamsa, Hiranyakasipu--to whom the Lord appeared in various incarnations just to kill them. Therefore God's mercy is shown to the asuras if they are fortunate enough to be killed by Him."
"The asuras sometimes become so powerful that they can engage even Narada Muni and similar devotees in their service. This does not mean that Narada was subordinate to Hiranyakasipu. Sometimes, however, it so happens in this material world that great personalities, even great devotees, can also be controlled by the asuras."
"Herein it is mentioned that Lord Siva is always accompanied by his material energy (saktya ghoraya). Material energy--goddess Durga, or goddess Kali--is always under his control. Goddess Kali and Durga serve him by killing all the asuras, or demons. Sometimes Kali becomes so infuriated that she indiscriminately kills all kinds of asuras. There is a popular picture of goddess Kali in which she wears a garland composed of the heads of the asuras and holds in her left hand a captured head and in her right hand a great khadga, or chopper, for killing asuras. Great wars are symbolic representations of Kali's devastation of the asuras and are actually conducted by the goddess Kali.
"Asuras try to pacify the goddess Kali, or Durga, by worshiping her in material opulence, but when the asuras become too intolerable, goddess Kali does not discriminate in killing them wholesale. Asuras do not know the secret of the energy of Lord Siva, and they prefer to worship goddess Kali or Durga or Lord Siva for material benefit. Due to their demoniac character, they are reluctant to surrender to Lord Krsna, as indicated by Bhagavad-gita (7.15)."
Bhaktivedanta Book Trust. Excerpted from various texts and purports of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.