It started slow; humanity grew, and with it, tools and machines emerged. They grew like weeds over the years, becoming more and more complex with each passing generation. At first, it didn't seem like these inventions could cause any harm; all they did was help humanity, pathing the way for the advancement and success of our species. That was our mistake; seeing only the silver lining of the cloud.
Then the damage started to show. Forests were depleted, and then completely wiped off the map as humans cut down their trees, leaving no time for the environment to heal. Some animals lost their habitats while others went extinct, the greed of humanity having destroyed their homes, scared off their prey, and left them starving, their numbers rapidly diminishing. Levels of greenhouse gases rocketed sky high; glaciers melted, water levels rose, then fell again as ponds, streams, rivers, and eventually even oceans were drained for water, fish, salt; anything humans could use. Smoke, methane, and exhaust from machines and cars polluted the air, lethally so in some areas. Trash and oil tainted whatever water that was left, killing marine life in droves. Food chains broke and collapsed, entire ecosystems were destroyed, and species went extinct, dropping like flies.
As Mother Earth suffered, so eventually did humanity; the increasingly warm weather quickly killed crops and other plants, along with a lack of water and pollinators. With animals already dying out, food became scarce. Pretty soon, few had the money to buy it; they ended up homeless, starving, and bankrupt. The powerful and wealthy were exempt, for a short while; they used their money to hold them up in a world that was crumbling around them. However, pretty soon that money wore away, and the rich and powerful were left just the same as everyone else. With so many people lacking the money to afford good food, water, or healthcare, disease plagued the land. Even the simplest diseases could now turn life threatening; a common cold could easily turn into pneumonia. These outbreaks, plus the severe lack of food and water available, quickly decimated the human population.
That's how humanity destroyed itself.