The objective here is to destroy the Earth, as specified in the article. Sure, there are lots of ways to kill everything on the planet, but it'll still be there when you're done.
If I've already roasted/pulverized/drowned everyone, who's going to complain?
And if a tree falls in the forest, will anyone hear it? As has been said the goal is destroy the earth. Not to leave no witnesses to confirm if the deed has been done.