Even if you believe in Taser International's pet medical diagnosis that only afflicts people in custody ("Excited Delirium")you would think this would be considered to be wrong.
Unfortunately, police commonly taser people in the US who are already in
handcuffs and in custody and it doesn't raise a hue and cry. They
usually say, as the officer in that video does, that they had to do it
for their own safety but it's quite clear it's because they are trying
to force "compliance." They lie because it's impossible to explain how
someone who is mentally ill, high on drugs or in the midst of having an
epileptic fit can be expected to understand the commands to "comply."
And frankly, when you listen to the agitated police screaming at the
person to stop struggling, it's quite clear that if "excited delirium"
exists it is a disease that afflicts the authorities as well.