Kalki avatar had nothing to do with world destruction. If the entire world was to get destroyed, it would never be doing of Kalki avatar. An avatar is always a messenger of peace.
The destruction of entire world is not a possibility in near future or many millennia henceforth.
But coming of an avatar always rings bells for the entire world community. The reason being, advent of an avatar is always preceded by a battle of Mahabharata magnitude.
The world is presently passing through difficult times with ethics and morality in the back burner. Such a situation cannot continue for long. The world is trying its best to pull on but for how long. In a short while from now, the delicate string that bonds world community will break. And world would witness a man-made calamity, a full-scale nuclear third world war.
The devastation in world war 3 would be catastrophic so much so that 1200 million people would lose their lives. And in such times humility looks forward to a savior, messiah… an avatar that could assuage their hurt feelings, provide a soothing balm.
Mounted with jnana wisdom of Bhagavad Gita and Upanishads (independent spiritual treatises) Kalki avatar would not only provide the requisite support to entire mankind but also usher the community towards Satyuga, the awaited golden period.
If the world war has to happen, it will. None in this world can stop it. The very fabric of nature has been crushed by mankind. One can witness humans stooping so low that it becomes difficult to differentiate human beings from animals.
In such times even an enlightened one like Mahavira, Gautama Buddha on Jesus Christ and Prophet Mohammed could not do much. Kalki avatar apart from being a spiritual person also possesses lethal powers of Chanakya (the most able administrator in the entire history of mankind). To save countrymen and soldiers from untold misery, Chanakya got 100 kings eliminated in one night. One can imagine the fury and restlessness with which Kalki avatar would operate. A ruthless person of the highest order would be the judgement of many.
As per Hinduism scriptures, to bring order to the society, to re-establish Dharma (righteousness) advents an avatar but an avatar is never instrumental in bringing about a world war especially of Mahabharata magnitude.