Qualm, scruple, compunction, demur can all denote a feeling of doubt or hesitation as to the rightness or wisdom of something one is doing or is about to do.
Qualm implies an uneasy, often a sickening, sensation that one is not following the dictates of his conscience or of his better judgment.
Scruple denotes mental disturbance occasioned by doubt of the rightness, the propriety, the fairness, or, sometimes, the outcome of an act; it often implies a principle as the source of the disturbance, and it may imply an overnice conscience or an extremely delicate sense of honor.
Compunction (see also PENITENCE ) implies a usually transitory prick or sting of conscience that warns a person that what he is about to do or is doing is wrong, unfair, unjust, or improper; it may additionally suggest a degree of concern for a potential victim.
Demur stresses hesitation to such an extent that it carries a stronger implication of delay than any of the other terms; it usually suggests, however, a delay caused by objections or irresolution rather than by an awakened conscience or by a scruple or compunction.