Devi Mahatmyam, which is a part of Markandeya Purana doesn't tell that Devi had any problem killing the Ratkabija. You can read the full story in this chapter. Actually the Gods (devas) were greatly afraid seeing the alarming increase in the number (clones) of Raktabija:
व्याप्तमासीत्ततो देवा भयमाजग्मुरुत्तमम्॥८८.५१
Asura’s blood pervaded the whole world; ...
Karkati is one such example. She takes boon from Brahma and realizes later to become good and becomes Jivan muktha.
Her story is mentioned in Part 3 - The Story of Karkaṭī in Utpatti-prakaraṇa of Yoga-Vasistha
Initially, she performs an intensive tapas and asks Brahma for a boon as follows
After she had passed thus a painful Tapas for
1,000 years, the Lotus-...
Well the story you are talking about is the story of a demon named Raktabīja, he came to known by this name after he was blessed with a unique boon by Lord Brahma. As per this boon, if a drop of blood from his body would fall on the ground, clones would be formed out of that drop. The story goes as follows:
Raktabīja was an asura (loosely translated as ...
Devas and humans used to reside together on Earth at some point in time.
In ancient times gods and men used to live together in this world.
Then the gods went to heaven by performing rites, while men were left
behind. Those among them who perform rites in the same manner get to
dwell in that world together with the gods and Brahman.
For this purpose ...
Most of the races are named after wives of Kaśyapa. For example,
Descendants of Aditi: Adityas (Devas)
Descendants of Danu: Danavas
Descendants of Diti: Daityas
Origin of Rakshasas is different which is described here.
Though Danavas, Daityas and Rakshasas are used interchangeably or collectively (as demon or asura) but they are different.
As per Śrīmad-...
There are some places in Vedas where Indra is shown as killer of asuras.
In Rigveda 6:26:3 It is written that Indra killed a demon called susna.
Rig Veda 6.26.3 Thou didst impel the sage to win the daylight, didst ruin Śuṣṇa for the pious Kutsa.The invulnerable demon’s head thou clavest when thou wouldst win the praise of Atithigva.
Indra also killed a ...
Battles between devas and asuras are described or at least referred to in the Rig Veda. However, we should take care to note that the Vedic asuras are divided between the benevolent Ᾱdityas (led by the Vedic Varuṇa) and the negative Dānavas (led by the Vedic Vṛtra). The main examples are as follows -
The elements of the famous battle between the deva Indra ...
Manu Smriti 12.60. He who has associated with outcasts, he who has
approached the wives of other men, and he who has stolen the property
of a Brahmana become Brahmarakshasas.
Can a human of degraded character become demon or rakshasa?
Yes, as shown by the verse given above. But obviously that happens in the next life.
Can a human of degraded character become demon or rakshasa? Or can a demon become human through his good character? Justify with refrence of scriptures.
Manu 12.39 - I am now going to describe, briefly, in due order, those migratory states into which one falls through each quality from among these.
40 - Those partaking of ‘Sattva’ reach the state of ...