Socinian unitarians love to throw out the execution of Michael Servetus in debates with Trinitarians. They maintain that John Calvin “brutally murdered” Servetus, and somehow (I have yet to figure out how) believe that this is an argument in favor of their position. Yeah, illogical nonsense, I know. But still they pound the drum.
If confronted with this argument by a unitarian, it is of course a good idea to point out the ad hominem fallacy and dismiss their argument as absurd. But to really take the wind out of their sails, nothing beats presenting the facts of the event with Servetus and Calvin and watching as the unitarian sparks and fizzles.
Here to give you the real history, is Dr. James White: