The Nature of Evil at the End of Kali Yuga
In all four yugas, the Supreme Lord incarnates to punish the evildoers of that era and re-establish Dharma. In Kali Yuga, the evildoers will surpass all limits of immorality and be called Mleccha.
In Satya Yuga, all human beings were well-versed in scriptures and followed Vedic traditions. However, sages and saints became arrogant and egoistic, leading to a decline of Dharma. Lord Maha-Vishnu took incarnations to re-establish truth, peace, compassion, forgiveness, and friendship.
In Treta Yuga, Lord Rama bestowed His mercy upon mankind. At the end of Treta Yuga, the Supreme Lord vanquished Ravana and many demons, followed by the Khand Pralay (partial annihilation).
In Dwapara Yuga, Lord Krishna killed Kamsa and many demons, guided the Pandavas in the Mahabharata war, and ultimately established Dharma. After Lord Krishna's departure, about 1,200 years of Kali Yuga passed during which the influence of Kali spread through hidden, sinister motives.
When Krishna left His mortal body and ascended to Golok Vaikunth, Kali Yuga had completed 1,200 years. After the demise of King Parikshit, Kali Yuga took its full form and spread its influence throughout the entire universe.
In Kali Yuga, people become dominated by greed, delusion, lust, anger, pride, materialism, and indolence. Those who:
...are called Mlecchas in the Kali Yuga.
During this period of desperation, the Supreme Lord Kalki will appear in His extremely ferocious form — like a fiery comet — upon the Earth to re-establish Dharma and annihilate all the sinners and evildoers known as Mlecchas, ushering in the pure Satya Yuga.