After the match Kidman gave Jackie a BK Bomb, now maybe it's because I view faces as heels and vice versa but Jackie was going to slap Kidman, so in the context of kayfabe what else was Billy supposed to do? Just stand there and get slapped like a punk? I'll admit when I first saw this as a kid I thought Kidman was the biggest dick in the entire WWE, to be honest I still think he was a good heel but he was sympathetic as well, he didn't want to do the SSP so he had concern for the safety of his fellow competitors.
Jackie wanted to keep Kidman safe. Kidman starts yelling at her so she attempts to defend herself and get away from him and he (A trained fighter, kayfabe wise) hits her with the move he's an expert in, knocking her unconscious.