You have a time machine with a single round trip. You hate everyone. And you are lazy. What is the least you can do to completely screw over the world with your one trip through time?
By eradicating one species, you're probably going to save the entire planet. I guess in 500 million years the descendants of modern crows could become the new dominant species and they'll end up nuking the planet sooner or later. You win some, you loose some.