Unregulated genetic engineering/biotech or unregulated nanotechnology.
Someone will develop a bacteria or nanomachine that does something useful or benign, but when it gets out into the wild, by accident or otherwise, it turns out the side effect is that it destroys all life.
Example: A bacteria that secrets ethanol as a waste product, but it gets out and it turns out it inhibits root function in plants and destroys all plant life, with animal life to follow shortly.
With nanotech you've got the grey goop doomsday scenario.
Technology will kill us.