I'm surprised it's as low as 25%

Depending on what your cut off point is for sadism that number could be a lot higher. There are also what could be called passive sadist, those that would not actively take a role in sadistic acts but are quite entertained by it. I suspect most people who watch bull fighting would be physically repulsed at the thought of actually stabbing a live animal with a blade, but they sure can watch it. Also, the popularity of videos griefing online gamer lend me to believe many people enjoy this type of mischief.