A person who is always dissatisfied
A). Heretic
B). Felon
C). Malcontent
D). Surrogate
Malcontent
One word substitute is Malcontent.
Heretic: a person believing in or practising religious heresy.
Felon: a person who has committed a felony.
Malcontent: a person who is dissatisfied and rebellious.
Surrogate: a substitute, especially a person deputizing for another in a specific role or office.