Mercenary, hireling, venal, hack are comparable though not closely synonymous terms when they are applied to persons, or their acts, services, or products with the meaning actuated or motivated chiefly by a desire for profit.
Mercenary stresses self-interest and often self-seeking as the guiding motive; it usually, except when applied to soldiers who serve a foreign power for a wage, applies to persons or services that should be prompted by altruism or by noble aims or should be characterized by unselfishness or selflessness.
Hireling suggests the attitude of one who serves for the wage involved or is guided by servile motives; the term usually, especially in its more common opprobrious use, implies a motive no higher than that of the reward promised or foreseen.
Venal implies purchasability. The term often connotes the use of bribery and nearly always carries a strong implication of corruption or of corruptibility.
Hack is used of a person, or of the work of such a person, willing to forfeit freedom of action and initiative or personal and professional integrity in return for an assured reward (as regular wages or political spoils). The term (and especially its corresponding noun hack ) commonly combines some of the implications of mercenary and venal; like the former it implies self-interest and like the latter corruptibility. Distinctively, hack regularly stresses the mediocre and uninspired quality of the person or his work and may imply a background of previous professional failure or inherent low order of ability.