All's fair in love and war. Cliched but true. Besides, what was fair about a slayer and a handful of wannabes going up against thousands of Neanderthalic vampires in the first place? But like you said, their plan never would have worked without that necklace. Even all those girls at full slayer strength wouldn't have been able to take on all those vamps. And seeing as how they had no idea what that necklace was going to do they were walking in knowing they were probably gonna die. I don't see how Buffy could have really thought they stood a chance.