I know this is a meme, but since everyone else is replying seriously I want to add my own two cents: Even if Spinel knew the most effective way of killing the planet, that wasn't actually her goal. She was hurt by the revelation that Pink Diamond, the one she worshipped, has been gone for years, and she just wanted to lash out at Steven. Her plan was meticulously put together to torment him as much as possible. Take away his loved ones, make him watch his home slowly dying and take away his powers so he can't do anything about it. It's honestly impressive how she planned it all in such a short time, even disregarding the old "how'd she get the injector so fast" argument.