In another words, could a Sith introduced himself to the Republic to have a peaceful life demanding protection from the Jedi if case arises?

Doubtful.  The Sith employed the Dark Side of the Force, drawing "power from raw emotions and feelings such as anger, hatred, greed, jealousy, fear, aggression, megalomania, and unrestrained passion." (Wookiepedia)  Given those characteristics, it's doubtful a Sith would pursue a "peaceful life."  Even if they attempted to mask their true selves and their intentions, attempt to portray themselves as benign, the violent history of the Sith would precede them.