As a reasonable reason why I'd destroy this planet is if humanity began to spread to other planets and started Terra forming them to suit them like Earth. I cannot stand humanity's natural instinct to bread and over populated an area then spread the disease that is humanity onto other planets. Life is able to grow just about anywhere, who knows Mars might be teeming with life in 150,000,000 years if humanity doesn't touch it.
It is ignorant to believe humanity has direct right over anything that exists so I'd destroy the world to stop any further madness of mankind.
My other reason: forget people, KA-BOOM.