Being king or queen does not exclude you from moral standard. There were decent kings or queens in the past(at least not as crazy). She didn't intentionally hurt her people before because she didn't even care to. When they were in the way then she was completely fine with getting rid of them. That was evil. If she was not evil, she would not let the legion prey on her people in the first place.
Cutting her people from the well made her people weaker thus much more weaker of a force against the demon.