Stefan Zweig’s relatively (in the grand scheme of things) book Castellio gegen Calvin: Oder, Ein Gewissen gegen die Gewalt, strives to answer that question (among others). The link provides a few reviews of the book that Calvin-ists might find engaging. The book was translated into English a couple of years ago (and is again at the forefront of discussion with the 500th anniversary of Calvin’s birth looming next year).
Oh and yes, for Zweig, Calvin was like Hitler.

According to many atheists all Christians are no better than Hitler so yes.
By: Drew on June 26, 2008
at 2:39 pm
[...] War Johannes Calvin ein Tyrann wie Adolf Hitler? [...]
By: Dies und das — soomah on July 2, 2008
at 10:47 am