Bellatrix burst into loud, appreciative laughter immediately, both at the student's fall and the expression on his face. The poor dear was so confused - of course, she felt only delight and no pity.
Roddy always knew when she needed a laugh. But she couldn't only laugh and not participate in tormenting someone. As soon as the boy shakily got back to his feet, she pointed her wand at him and shouted, "Tarantallegra!" She did not care about getting caught, and there were no professors around anyway.