It still sounds like manslaughter to me.
I’m not certain about this, but I believe that it would have to be shown in court that the officer believed that he indeed was applying the current.
According to his statements and an investigation by the department, since the suspect didn’t react in the usual manner, the officer may have truly believed that nothing was happening.
In that case, it would be an accidental death, not manslaughter.
When the justice system is committing the injustice, it’s time for vigilantism IMO. I have always been an “eye for and eye” sort of guy. Perhaps it would make an impression on them if one of their kids were elecooctuted (by accident, of course). If they didn’t have any kids, then maybe a wife or two nieces or nephews...