Scenario 1: Steven, and thus Pink Diamond, dies or is at least poofed, making for easy prey. Injector stuff goes as planned. Mission accomplished. Revenge has been exacted.
Scenario 2: Steven does not die/poof, as she expects ("You don't poof? Figured as much"), and even if he overpowers her, even shatters her, it won't matter because the world will be rid of all organic life (including Steven?) anyway, and all of Pink Diamond's friends will be dead or reverted to factory specs. Revenge exacted.
Scenario 3: Steven does not die, he overpowers her WITH the rejuvenator (as happened) and not only will all organic life be gone and Pink Diamond's friends dead or reverted, but Spinel will go back to being happy again whenever she's reinitialized. Revenge exacted and a new lease on life acquired.
If it weren't for the whole memory restoration thing, she would have gotten what she wanted no matter what happened. Even if Steven restored everyone else's memory and even stopped the injector, but left Spinel as a happy go lucky fool, who cares, she'd at least be happy again.