We have so many people on this planet with the brain capacity of a 10 year old. They do not realize the true greatness of the human species. They are ignorant and the government should create a deadly pathogen to wipe these people out. I am a terrible person and this is just for fun.
Um, I realize that not everybody's perfect, but as long as they're not running around murdering everyone in large groups then they're committing no crime and they deserve to live. The word "should" does not apply, in my opinion, in this case. No one "should" have to die for anything, in the best case, and if we can figure out a way to solve the world's problems without destroying a good chunk of the world, then we know we're the good guys as opposed to, say, the opposite. The more we can improve our world with regard to morals and conserving life, the more we're getting the most of both sides, i.e. efficiency and ethics. Thus, our society, acting on both of these principles, would be closer to perfection. I don't think it's humans' responsibility to decide who should die without threat to themselves.
The opinion-asker mentions plagues, similar to that of the Black Death or Smallpox. These diseases killed perhaps 1/3 of infected, and far more in mutated forms; nowadays, disease would spread much faster and mutate far more often, leading to an even higher death toll and death rate. Healthcare might slow it down, but only if enough doctors aren't infected, too, and the medical and transport industries are quarantined from infection. Assume perhaps 3 billion deaths and societal collapse for a generation or three, leaving the world in an impoverished state. Pretty picture, huh.