In the Bhagavad-gītā (16.7-20) the Supreme Lord has exhaustively described the nature of such atheistic demons:
Those who are demoniac do not know what is to be done and what is not to be done. Neither cleanliness nor proper behaviour nor truth is found in them. They say that this world is unreal, with no foundation, no God in control. They say it is produced of sex desire and has no cause other than lust. Following such conclusions, the demoniac, who are lost to themselves and who have no intelligence, engage in unbeneficial, horrible works meant to destroy the world. Taking shelter of insatiable lust and absorbed in the conceit of pride and false prestige, the demoniac, thus illusioned, are always sworn to unclean work, attracted by the impermanent.
They believe that to gratify the senses unto the end of life is the prime necessity of human civilization. Thus until the end of life their anxiety is immeasurable. Bound by a network of hundreds and thousands of desires and absorbed in lust and anger, they secure money by illegal means for sense gratification. The demoniac person thinks, 'So much wealth do I have today, and I will gain more according to my schemes. So much is mine now, and it will increase in the future, more and more. He is my enemy, and I have killed him, and my other enemies will also be killed. I am the lord of everything. I am the enjoyer. I am perfect, powerful, and happy. I am the richest man, surrounded by aristocratic relatives. There is none so powerful and happy as I am. I shall perform sacrifices, I shall give some charity, and thus I shall rejoice.' In this way such persons are deluded by ignorance. Thus perplexed by various anxieties and bound by a network of illusions, they become too strongly attached to sense enjoyment and fall down into hell.
Self-complacent and always impudent, deluded by wealth and false prestige, they sometimes proudly perform sacrifices in name only, without following any rules or regulations. Bewildered by false ego, strength, pride, lust, and anger, the demons become envious of the Supreme Personality of Godhead, who is situated in their own bodies and in the bodies of others, and blaspheme against the real religion. Those who are envious and mischievous, who are the lowest among men, I perpetually cast into the ocean of material existence, into various demoniac species of life. Attaining repeated birth amongst the species of demoniac life, O son of Kuntī, such persons can never approach Me. Gradually they sink down to the most abominable type of existence.
These verses from the Gītā aptly describe the demoniac nature.
There have always existed two types of men, the devotee and the demon. Long ago there lived a big demon named Rāvaṇa, who disguised himself as a sannyāsī, a renounced mendicant, and tried to steal the wife of the Supreme Lord, Rāmacandra. She was the goddess of fortune, Sītā-devī. In this way the demon brought about his own destruction.
