User blog comment:FairyRaven/Fairy Tail Chapter 365: Fairy in the Jail!, Review and Commentary/@comment-24257713-20131229050248/@comment-7418318-20131229180625

It's not necessary for Erza to know that Kyouka is unwilling to seriously injure Mirajane.

Yes there is a chance that it may shake Erza's confidence, but that's a rather small chance unless Kyouka follows her threat up with some actual action. And if Erza does not cave in just from the threat, not providing the proof that Kyouka is serious will mean the entire attempt will backfire.

Would you take an action that has a little chance to work at all and will only get you a little benefit with a at least moderate risk of backfiring or would you go straight to the method in which you have full confidence of it working?

If the person to decide was Human or considering Humans as equals, perhaps due to moral reasoning, but Kyouka is inhibited by neither of those things. She's not Human, she considers Humans barely above animals at best and she is utterly immoral and sadistic.

Trying to scare Erza into revealing information without torturing her would hint at a mercy or lack of resolve that is simply not present in the Demons. As Sayla said: "In the stories of Demons, there is no such thing as mercy." and that scene with Erza is getting that across point-blank, unobscured in all it's horror. Sure it is disturbing, but that just makes it all the more effective as a story device, because it invokes emotion in the reader.

As I said before, I prefer this over Naval Battle, since here it's a Demon who's torturing a Human for very important information that may even decide the fate of the entire world, rather than a Human doing it to another Human for petty reasons.