Malcontent

Example:He is known as a complainer who never stops spreading negativity among the team.

Definition:A person who frequently complains about minor inconveniences or about things in general; a persistent troublemaker.

Example:She is dissatisfied with the lack of recognition for her hard work.

Definition:Unhappy because of some unsatisfactory condition or circumstance; discontented.

Example:The dissentionist quickly spread dissent among the group, causing division.

Definition:A person who belongs to or supports a faction in a movement, organization, etc., that is opposed to the others; a dissenter, especially a political dissenter.