I am getting very skeptical about the home-made bioweapon that ends the world.
It isn't unreasonable to think that some lone idiot could make a new version of smallpox or bubonic plague or bird flu that goes the distance. My question is how in heck would they test it? DNA is like the worst imaginable spaghetti code, so it isn't like you just flip this one sequence here and your ordinary flu bug is 99 percent fatal. And if you combine in other stuff you have no real idea what unintended side effects might make your world-killer fizzle out. And given the very large number of angry people with guns who would be looking for me, I would want to be DAMN sure that my world-killer would really kill the world.
If I wasn't suicidal, I'd also want a vaccine. How are you going to make that vaccine without testing? I mean like really infect people, vaccinate some uninfected people, put them together, and see who dies. And for your potential world-killer to go the distance, it would have to be easily transmittable -- so that implies that you would need wicked good biocontainment and someplace very private to do your evil deeds.
Now, there are still some awful things you could do without needing to worry about testing so much. Making a hypothetical virus that would be asymptomatic (or just very mild) for nearly everyone except some small group with specific DNA markers, or just one person, would be possible. It would sure suck to be president or even a university professor who gave the wrong little snowflake a shitty grade.