Even if you have a possible justification for doing so — ’cause it might be illegal. From the ABA Journal:
Proceedings have been delayed in a California misdemeanor case in which the defense is claiming that police brutalized their client with a stun gun during his arrest at a shopping mall last year.
That’s because the defense team is now being criminally investigated for allegedly violating human experimentation laws by repeatedly using a stun gun on their client themselves during an evidence-gathering effort in a law office.
Additional details here. Our tipster, a criminal defense lawyer, observes:

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“I can’t decide which I like better:
(1) imagining those nervous, sweaty-palmed, study-group types from law school, wringing their hands and saying, ‘C’mon, guys, we have to be PREPARED! How are we gonna know what he looked like when he was writhing in agony unless we shock him AGAIN?’ or
(2) the idea of defense lawyers seizing the opportunity to taser a client — which we have ALL dreamed of doing.”
Shocking Case: Legal Team Tasers Client [ABA Journal]