https://en.wikipedia.org/wiki/Optimum_population
>The optimal world population has been estimated by a team co-authored by Paul R. Ehrlich.
>Based on this, the estimation of optimum population in the 1994 study was to around 1.5 billion to 2 billion people.
>A 2014 study published in the Proceedings of the National Academy of Sciences of the United States of America concluded that, with a fertility-reduction model of one-child per female by 2100, it would take at least 140 years to reduce the population to 2 billion people by 2153.

What do you think the most effective way to artificially kill billions of people (5 billion at max) would be? Keeping in mind that if you were to kill billions of people at once, the world probably would not have enough graves for them.
One way to do it could be deploying billions of nanobots and accomplishing this over years or so but that sounds very much in the realm of sci-fi, right now at least. Artificially created virus might also be a good choice, considering the current situation.
Additionally, a random massacre of the human race would be a bad idea since ideally you would want the human race to flourish after the purge, to that regard, what metric do you think would best determine the "worth" of a human being?