Eh. Even outside the meta consideration that Cheetah is not going to die here, this is pretty much a clear bluff on Diana's part. She needs to know where Etta is. Only Minerva knows where Etta is. If Minerva is alive, she might let it slip through carelessness or just for the hell of it, or Diana might be able to get something out of her some other way. If she's dead, she's a dead end. Her death serves no purpose re: Diana's primary goal, while her continued existence offers a slim possibility of hope.
Which means the only way Minerva ends up dead in this situation is if Diana both does not care about the slim hope of getting the info that could save Etta out of her, and is vindictive enough to kill Minerva just for setting this situation up. Which isn't going to happen - Diana can't give up any hope of finding Etta, and Diana wouldn't kill for the sole sake of vengeance (and killing Minerva doesn't serve the greater good either, since she's proven herself capable of reform and antiheroism in the past).
It's a bluff. Diana may not even realize it's a bluff when she says it, but it's a bluff.
That said, I must admit I'm kind of digging the neck-snapping meme. It's the perfect form of execution for an amazon - it demonstrates a significant power differential between killer and victim, and is among the quickest and most humane deaths possible. I really would not mind at all that being a thing she occasionally does (the way she occasionally has in the past - Poseidon's son springs to mind).