Whether or not Bellatrix really thought that Pansy should be tortured, it didn't really matter much. Anyone should know not to go against the Dark Lord. Even if it meant watching someone they love suffer. It would only end in a lot more pain, and possibly someone would be dead. She'd been watching, though not the actual torturing. She'd been watching Fletcher and Draco's face. To see what they felt, what it was like for them. That was almost better than hearing Pansy squeal. She liked the girl well enough, but she liked torture much more.
Draco's movements weren't unnoticed, and when she spotted Draco go for his wand, Bella seized the opportunity to prevent him from doing something stupid. She casted a quick expelliarmus on him, though with her own wand it looked like the wand might crack and break under the pressure. Oops. It was still much safer than him trying to get one over on his Dark Lord.