I'm speechless.

Far too many cops are sadistic bastards.
There needs to be much tougher psychological screening of recruits for this profession

Cops everywhere can be problems. The job really needs people who are smarter and better trained. Anyone who is smarter and better trained wouldn't want to be a cop. Remuneration / pay.
Still, it is not possible to excuse cruelty such as this. The humiliation would have probably been worse than the shocks.

Taser abuse by police is kind of a U.S. epidemic. I think most cops are standard issue humans that have been put in a very adversarial position, as in us vs. "them". Until we change our practices of how we treat one another, we are going to see more incidents like this.