No the real reason was that if Servetus had been acquited (under Geneva Law) it would have been Calvin who would have had to face the funeral pyre for bringing false charges.
By appointing his secretary to be the accuser and having his secretary sign the indictment and suffer the imprisonment, Calvin was able to ensure that the only accuser at risk of the fire would have been his secretary and not him.
Pretty noble of Mr. Calvin, eh?
;)
Jean