Yeah its definitely carrows for me. Like you said, Carrows used Crucio on students for the fun of it, and attempted to make students use it on First Years. Umbridge only used the blood quill on students who she thought was "misbehaving", and of course the blood quill isn't as bad as Crucio. Even if she did try to use it on Harry, at least she was just a bit hesitant on using it. And, for Lockhart, yes he didn't actually teach the students anything and was a huge fraud, but I mean he didn't purposely harm the students (unless they found out about him being a fraud), soooooo yeah.
