Had she, in fact, killed Negan with the one bullet in the one gun that Team Family had, then the Saviors would have mowed down a lot more than just Olivia.
The Walking Dead has given us a situation already where Rosita could have taken Negan out at close range, which was referenced tonight during her confrontation with Father Gabriel.
The show likely won’t lob out another freebie sneak attack option like that again, but even if it did, the singular goal of killing Negan would probably be outbalanced by the deaths of all the other Alexandrians.
She would love to have that moment back, saying that it would mean the now-flippant Eugene would still be there and Olivia would still be alive, but that’s definitely not provable.