He only voted for the candidate because he was paid to do so. He's just an unscrupulous person/man.
Dictionary.com provides the perfect description and meaning of this adjective
Unscrupulous, unprincipled refer to lack of moral standards or conscience to guide one's conduct. The unscrupulous person is without scruples of conscience, and disregards, or has contempt for, laws of right or justice with which he or she is perfectly well acquainted, and which should restrain his or her actions: unscrupulous in methods of making money, in taking advantage of the unfortunate.The unprincipled person is without moral principles or ethical standards in his or her conduct or actions: an unprincipled rogue; unprincipled conduct.
If the OP is looking for a single noun, rogue, would be appropriate but it does imply that the person is generally dishonest and untrustworthy. Alternatively, an opportunist, is a single word and a noun.
1. a person who practices opportunism, or the policy of adapting actions, decisions, etc., to effectiveness regardless of the sacrifice of ethical principles: