A Taser is not designed ("entire idea") to injure or kill with electricity. It's designed to temporarily incapacitate, with a side effect/risk of injury or death.
"Temporarily incapacitating" someone in a wet, slippery area with very hard surfaces seems like it might have a high risk of going wrong. Article even says he had broken ribs.
Or maybe it's designed to inflict as much pain and humiliation as possible without permanent harm. Because that's how it's actually used by the police, to punish their victims for disobedience.
