A savior messiah, avatar is not about killing anybody but spreading message of peace world over. Some may say this could be accomplished by an enlightened one like Mahavira, Gautama Buddha even Jesus Christ and Prophet Mohammed. Yes, spreading message of peace is within purview of enlightened ones but what about re-establishing dharma (righteousness) against all odds. This feat can only be accomplished by an avatar.
The only purpose of advent of 10th avatar of Vishnu Kalki Avatar would be to bring order in the society. In the aftermath of world war 3 everything would go haywire. Even the best of human beings would not be able to realize, think what to do. It is then precipitates jnana wisdom of Bhagavad Gita and Upanishads (independent spiritual treatises) that acts as a soothing balm to the ailing society.
A drastic change in the perception of human beings is brought about by Kalki Avatar. Before advent of Kalki Avatar or start of world war three people gave preference to dictates of ego thereby releasing material riches all the time. With advent of Kalki avatar the material inclinations take a back seat forever. People become spiritual overnight (not religious).
If over 1200 million people get killed in world war 3, it would not be doing of Kalki avatar but in the aftermath of world war 3 it is only Kalki avatar that can show streak of light to the devastated humanity. Upholding Dharma (righteousness) is not a small task when above 5 billion people world over gloated in worldly riches almost all the time.
Kalki avatar would kill the animal insect of human beings for long times to come. So much so that in the aftermath of world war three mankind witnesses start of Satyuga, the golden period.
Human beings remaining the same only the thinking pattern and tendencies change from materialistic to spiritual. Earlier about 95% people were materially inclined and 5% spiritual, in the aftermath of world war 3 it will be a total reversal, 95% people want to practice spirituality and hard-core 5% still indulged in religious or malpractices.