I am under impression that the first syllable in a word gets a dakuten when it is transformed into voiced form under the effect of rendaku. But in the idiomatic expression 様みろ {ざまみろ}, the kanji 様 which is read as さま gets voiced and obtains the dakuten without any rendaku. How did that happen? Apart from saying "well, it's a set phrase so just accept it as is", could there be any reason, etymologically speaking, why it became ざま?
Also, are there any other scenario where words get voiced/dakuten without rendaku?