The basic meaning of Sura is from Saur/Surya i.e. one who radiates with light, just like a sun and knows his real self, Aatma(which is without Tamas). Hence, Gods in Hinduism are always depicted with a Sun like halo behind their heads. Asura is the opposite, one who does not know the Aatma and hence full of Tamas. Difference between Asuric and Sura buddhi has been discussed number of times in Vedas.
From Chandogya Upanishad, Chapter 8.7-12
The Story of Indra and Virochana
Once Prajapati, the creator of the universe announced, “The Self is
the sinless, ageless and deathless One; it has no sorrow nor hunger
nor thirst. The goal of all its desire is the Truth, Truth is the one
thing worthy of its resolve. It is this Self that has to be sought
after, it alone one should seek to know. And one who seeks after the
Self and knows it, gains possessions of all the worlds, wins all that
is desirable.” Both the gods and the demons heard this announcement
and thought, “We must know this Atman or Self and seek this knowledge
that promises all worlds.” Hence, Indra, the king of the gods and
Virochana, the leader of the demons, approached Prajapati with all the
humility of a disciple and requested him to impart this knowledge to
them. Prajapati accepted them as his disciples and asked them to stay
with him for thirty-two years. Both, Indra and Virochana, stayed on
living the life of a Brahmachari. At the end of that period,
Prajapati called both of them and said, “Dear ones, the person visible
in the pupil of the eye is Atman. This Atman is the Brahman, the
immortal and fearless. Go and see yourself in a pan of water and then
tell me what you saw.” Next day when Prajapati asked both of them to
narrate their experience they both said, “Lord, we saw the Self
entirely as we are, the very image even to the very hairs and nails.”
“Go and adorn yourself and then look into the pan of water and tell me
about your experience” said Prajapati to both of them. Happy, they
went and adorned themselves, looked into the pan of water, came back
and replied to Prajapati, “Revered Sir, we saw ourselves well dressed,
well groomed and well adorned.” “Very good, this is Atman, the
immortal and fearless, the Brahman” thus confirmed Prajapati to them.
Both Indra and Virochana, went happy and very much satisfied in their
heart. Prajapati saw them going away and exclaimed to himself,
“Whether they are gods or demons they will inevitably perish if they
are satisfied with the mere reflection of the Reality.” Virochana went
to the demons and taught them this knowledge by saying that this
bodily self is all that we need to know and serve and glorify. This is
the supreme knowledge by which one obtains the world here and
hereafter. Indra was different from Virochana. He realized that
something is wrong with what he has just learnt about the Atman. He
felt uneasy. He thought to himself that as the water reflects a well
adorned body so does it reflects a blind man if he is blind, a
defective man if he is defective. If a body is destroyed or dead, then
there is no reflection of a living body. How then could the body or
its reflection be said to be the deathless Atman? He went back to
Prajapati and said, “Lord, I see really nothing worthy in this body or
in its reflection. Please give me the true knowledge about the Atman.”
Prajapati was happy with Indra and asked Indra to stay with him for
another thirty-two years. At the end of that period Prajapati called
him and said, “That which moves about in dreams is Atman.” Indra
seemed to be satisfied and made his way to the abode of gods. But as
he went along, again doubt crept into his mind and he thought, “How
can the dream self be Atman? In dreams one suffers, one is wounded,
one weeps when afflicted. The fearless and sinless Atman cannot be
subject to such happenings. This does not seem to be the truth. Even
this time also something again is wrong with what I have learnt.” He
came back and prostrated himself before Prajapati requesting him to
explain the truth of Atman. As usual, Prajapati told him to stay with
him for another period of thirty-two years. Indra was all prepared.
At the end of this period, Prajapati called Indra and said: “The self
who is full pleased and is in a happy mood in deep sleep is the
Atman.” Indra was satisfied and went home. But before he reached his
place, again doubt overwhelmed him and he said to himself, “In a
dreamless sleep, the self does not know itself. It is not conscious of
its own existence. How then can the unconscious self be Atman when it
is described as the ever luminous one, ever conscious one?” He came
back again to Prajapati and reported to him about his difficulties.
This time Prajapati was more pleased with Indra and told him to spend
five more years with him. After the period was over Prajapati called
Indra beside him and said: “O Indra, you have deserved the knowledge
of the highest truth by your persistent effort and intense
inquisitiveness.” Saying this Prajapati revealed that part of the
highest knowledge to Indra for which he was ready at that point of
time. “This body is subject to death yet it embodies the deathless and
bodiless Atman. This embodied Self falls into the trap of all
dualities like pleasure and pain, but the bodiless Atman is not
touched by any duality. So long as the Atman resides in the body and
attaches itself to them he seems limited and restricted, but again
when freed from the body becomes one with the infinite spirit. When
the Atman leaves the body, goes wandering freely in the infinite
worlds. The eye, the ear, the senses, the mind are there only in order
that the Atman may see and hear and think. It is on account of Atman
and in the Atman that the things and beings exist. He is the Truth and
the final repository of all existence.” This time when Indra went back
to impart this knowledge to the gods, he had no doubt. The gods
received the true knowledge and therefore live in the full knowledge
of their Self. And because Virochana was satisfied with the knowledge
of body as self, the Asura demons, even today live in the ignorance taking
body as Self.
This is also discussed in Bhagwat Geeta, where "Asura" has been translated as demon in english, but the real meaning of word "Asura" is the one in whom Tamas dominates and devoid of knowledge of Self-soul, Satva and considers manifested reality is the only truth just like animals.
BG 16.1 – 16.3 The Supreme Divine Personality said: O scion of Bharat,
these are the saintly virtues of those endowed with a divine
nature—fearlessness, purity of mind, steadfastness in spiritual
knowledge, charity, control of the senses, performance of sacrifice,
study of the sacred books, austerity, and straightforwardness;
non-violence, truthfulness, absence of anger, renunciation,
peacefulness, restraint from fault-finding, compassion toward all
living beings, absence of covetousness, gentleness, modesty, and lack
of fickleness; vigor, forgiveness, fortitude, cleanliness, bearing
enmity toward none, and absence of vanity.
BG 16.4 O Parth, the qualities of those who possess a demoniac nature
are hypocrisy, arrogance, conceit, anger, harshness, and ignorance.
BG 16.5 The divine qualities lead to liberation, while the demoniac
qualities are the cause for a continuing destiny of bondage. Grieve
not, O Arjun, as you were born with saintly virtues.
BG 16.6 There are two kinds of beings in this world—those endowed with
a divine nature and those possessing a demoniac nature. I have
described the divine qualities in detail, O Arjun. Now hear from me
about the demoniac nature. Bhagavad Gita 16.7
Those possessing a demoniac nature do not comprehend what actions are
proper and what are improper. Hence, they possess neither purity, nor
good conduct, nor even truthfulness.
BG 16.8 They say, “The world is without absolute truth, without any
basis (for moral order), and without a God (who has created or is
controlling it). It is created from the combination of the two sexes,
and has no purpose other than sexual gratification.”
BG 16.9 Holding fast to such views, these misdirected souls, with
small intellect and cruel actions, arise as enemies of the world
threatening its destruction.
BG 16.10 Harboring insatiable lust, full of hypocrisy, pride and
arrogance, the demoniac cling to their false tenets. Thus illusioned,
they are attracted to the impermanent and work with impure resolve.
BG 16.11 They are obsessed with endless anxieties that end only with
death. Still, they maintain with complete assurance that gratification
of desires and accumulation of wealth is the highest purpose of life.
BG 16.12 Held in bondage by hundreds of desires, and driven by lust
and anger, they strive to accumulate wealth by unjust means, all for
the gratification of their senses.
BG 16.13 – 16.15 The demoniac persons think, “I have gained so much
wealth today, and I shall now fulfill this desire of mine. This is
mine, and tomorrow I shall have even more. That enemy has been
destroyed by me, and I shall destroy the others too! I am like God
himself, I am the enjoyer, I am powerful, and I am happy. I am wealthy
and I have highly placed relatives. Who else is equal to me? I shall
perform sacrifices (to the celestial gods); I shall give alms; I shall
rejoice.” In this way, they are deluded by ignorance.
BG 16.16 Possessed and led astray by such imaginings, enveloped in a
mesh of delusion, and addicted to the gratification of sensuous
pleasures, they descend to the murkiest hell.