If I were looking to do something in the US to get rid of undesirables, I would:
1. Create a scare about a disease that will never really materialize.
2. Encourage people to get a vaccine for the disease. The vaccine, however, is really for a highly contagious, secretly engineered virus.
3. Give it some time, then release the engineered virus to kill the unvaccinated.
Way better evil plot than executing people at FEMA camps. Also, the nutjobs self-select, saving you the time and trouble of dealing with tracking anyone.