People are of two types. Swami Vivekananda said: "This world may be said to be divided between persons of demoniacal nature who think the care-taking of the body to be the be-all and the end-all of existence, and persons of godly nature who realise that the body is simply a means to an end, an instrument intended for the culture of the soul" (Complete Works, Vol. 3, Pages 77-78).
Bṛhadāraṇyaka Upaniṣad (1.3.1) says: "The offsprings of Prajāpati (Creator, the Lord of beings) were indeed twofold: the devas (gods) and the asuras (demons). Therefore (naturally), the devas (were) fewer in number, (and) the asuras (were) numerous. They (devas and asuras) rivalled with each other."
According to Swami Sivananda, the philosophy of good and evil is explained allegorically, in this mantra. All beings are of two types, devas and asuras. Devas are virtuous, and the asuras are directed merely by secular goals. Both of them rival with each other for the ownership of the worlds.
Since the beginning of creation, there have been the good and the virtuous, as well as the bad and the vicious. Devas represent the virtuous in whom the quality of sattva is predominant, and who have purified thoughts and refined actions. Asuras represent that particular class or nature of people, which is influenced and goaded by the twin forces of rajas and tamas, and which delights and takes pleasure in purely selfish affairs. This class or nature is very much opposed to the other one.
The devas and the asuras are not different from human beings. These twin forces exist within every human being and come to express themselves through the medium of speech and other organs of action and knowledge. When people are immaculate under the influence of pure thoughts and right actions, they become devas, the shining ones. When people are impelled by vicious thoughts, directed merely for one's own petty ends and sense indulgence, and opposed to the devas, they are called asuras. Devas perform only those actions that do not oppose the general conduct. The asuras base their actions on perception and inference, and are goaded by something or other which is not of permanent moral value.
The devas are outnumbered by the asuras who are numerous. Goodness is very difficult and rare to achieve, whereas vice and irreligiousness are always rampant. It is because the senses have a tendency to go out into the gross, visible world of objects, and not to see what is internal and spiritual. This is the reason why the asuras surpass the devas in number.
Sattva is a spiritual and divine quality. When it preponderates, righteousness and virtue prevail in creation, and truth, purity, non-violence and other divine qualities are practised by people. There is then absence of wrath, anger, lust, jealousy and other āsurika (demoniac) qualities. People become good and sweet-natured, tranquil and simple. They do not act merely to achieve secular ends and for their own individual profit. Vulgarity and sensuality become extinct. Those who are sāttvik believe in things spiritual and not in enjoyments that are visible and sense-engendered. They know that the things based on sense-perception and inference are not lasting, and that they delude the senses, while matters spiritual have a permanent value. Sattva expresses itself through good thoughts, good speech, good hearing, good seeing, good smelling, good touching, and good deeds, while rajas and tamas have their field of operation in the gross and sensuous activities, viz., lust and the like.
Rājasik and tāmasik people are attached to things mundane. They indulge in sensuality, vulgarity, wrath, passion and evil thoughts. Sattva stands for purity, rajas for hectic activity and tamas for total ignorance. Asuras have a good following, because the majority of people are impelled by the force of ignorance, to perform vicious actions for self-indulgence. The rise of rajas and tamas (asuras) is solely due to the mentality of human beings that has grown grosser and grosser, in the midst of the external objects of sense-enjoyment. Devas have limited followers, because they condemn sense-enjoyment which is very pleasing to all, and practise only those thoughts and actions that are right and just.
A regular warfare is going on in the world between these two forces, devas and asuras, good and evil. Whenever an individual is inclined to cultivate good thoughts and good actions, there emerges the deva in her or him, and manifests itself through her or his entire organs. Similarly, when an individual is led by sense-perception and sense-enjoyment, and is inclined to indulge in vices, there manifests the asura in her or him. When the former is prevailing over the individual, the latter is subjugated. Similarly, when the latter gets the upper hand, the former lies dormant. Since the dawn of creation, these two forces of nature have been vying with each other. Sometimes sattva prevails and people become pure and pious. At other times, rajas and tamas prevail, and there is preponderance of demerit resulting in degradation. Devas have been standing in combat with the asuras. It has been named as deva-asura-saṅgrāmaḥ, battle between devas and asuras. This warfare is continuing even to this day. Sometimes, divinity reigns supreme in the individual and the society, and at other times demoniacal nature catches hold of the world and rules, till it is ousted by divinity (Swami Sivananda, "The Brihadaranyaka Upanishad: Sanskrit Text, English Translation And Commentary," Pages 30-33).
Bhagavad-Gita Chapter 16 details the characteristics of the two types of beings in this world, the daivika (divine) and the āsurika (demoniac). Fearlessness, purity of heart, steadfastness in knowledge and Yoga, almsgiving, control of the senses, yajña, studying the Śāstras, austerity, uprightness, non-injury, truth, absence of anger, renunciation, tranquillity, absence of calumny, compassion to beings, uncovetousness, gentleness, modesty, absence of fickleness, boldness, forgiveness, fortitude, purity, absence of hatred, absence of pride: these belong to one born for a daivika state. Ostentation, arrogance and self-conceit, anger as also harshness and ignorance, belong to one who is born for an āsurika state. The daivika state makes for liberation, the āsurika for bondage. The persons of āsurika nature know not what to do and what to refrain from; neither is purity found in them nor good conduct, nor truth. They say, "The universe is without truth, without a (moral) basis, brought about by mutual union, with lust for its cause; what else?" Holding this view, these ruined souls of small intellect and fierce deeds, rise as the enemies of the world for its destruction. Filled with insatiable desires, full of hypocrisy, pride and arrogance, holding evil ideas through delusion, they work with impure resolve. Beset with immense cares ending only with death, regarding gratification of lust as the highest, and feeling sure that that is all; bound by a hundred ties of hope, given over to lust and wrath, they strive to secure by unjust means hoards of wealth for sensual enjoyment. They say, "This today has been gained by me; this desire I shall obtain; this is mine, and this wealth also shall be mine in future. That enemy has been slain by me, and others also shall I slay. I am the lord, I enjoy, I am successful, powerful and happy. I am rich and well-born. Who else is equal to me? I will sacrifice, I will give, I will rejoice." Thus deluded by ignorance, bewildered by many a fancy, covered by the meshes of delusion, addicted to the gratification of lust, they fall downwards. Self-conceited, haughty, filled with the pride and intoxication of wealth, they perform yajña in name, out of ostentation, disregarding ordinance. Possessed of egoism, power, insolence, lust and wrath, these malignant people hate the real Self within, in their own bodies and those of others.
How to become a deva? "The touch of the soul can paint the brightest colour even in the dingiest places; it can cast a fragrance over the vilest thing; it can make the wicked divine — and all enmity, all selfishness is effaced. The less the thought of the body, the better. For it is the body that drags us down. It is attachment, identification, which makes us miserable. That is the secret: To think that I am the spirit and not the body, and that the whole of this universe with all its relations, with all its good and all its evil, is but as a series of paintings — scenes on a canvas — of which I am the witness" (Swami Vivekananda, Complete Works, Vol. 2, Page 37).