She'd already cut Cheetah's face to get her point across.
Even if she stopped right there, she's already wounded a captured and defenseless Cheetah in order to get Cheetah to talk. The torture already started.
And I'm okay with that. I do think it's a remarkably dangerous "slippery slope" but many things heroes do fall into that category. I think most of the time things we call "torture" are unjustified, and unlikely to yield results anyway.
But I do allow for common sense, and I think it would be a poor stance for the "Avatar of Truth" to take, saying it's cut and dry/black and white. The truth of the matter is that things get complicated out there.