Not so easy, they won't kill everyone, they want first total control (that's what the vaccine is about, taking away legally the last crumbs of freedom) and to reduce specific types of target; look it from this point of view, they need healthy people who can work, produce income, that thinks to be alive and free, living in their system, so that the cycle can continue and this people can pay taxes, consume products, waste the few bucks they hardly can keep by the end of the month, and so on, that's the perfect success for them

Contrary, if you just start to kill everyone randomly, or if you kill 90 percent of the population, you risk to keep old survivors, kids who currently are an expense and can't produce income, and so on, their system would fall quickly as there would be almost noone left and no luxury / consumes and stuffs, would backfire them forever

remember, they never intended to leave gold / assets / whatever behind, they want every last resource available, so the factory has to be running smoothly (factory -> slave work / income / taxes / system etc as up), and at the same time, spiritually, if we can definite it this way, they also need to feel our pain, anger, suffering, and any other kind of bad feeling we can produce, that's how they get satisfied, if you take out those two things, what would be left for them? nothing, they would be rulers of an empty planet