Negative utilitarianism is so silly, happiness and suffering are absolutely subjective and multiple points of perception can't be outweigh because they are all independent and anyone reading this is just one more.
So everyone is suffering from this world except one man, the only reasonable thing is to end the world. Problem is that this conclusion would come from a suffering individual wish the only thing he haves to do to end the world is to kill himself; wish is completely unrelated with the faith of the world perceived by everyone else.
Not to mention the issue with current time, meaning an individual feeling severely depressed for 60 years that today somehow feels extremely happy will say that is not so bad, because emotions are blinding and are usually loosely attach with logic specially statistics.
The other thing is that there should be a way to end the world without provoking more suffering than it will get attempting it for negative utilitarianism to have real-life applications.