Objection 1. It would seem that servile fear is not good. For if the use of a thing is evil, the thing itself is evil. Now the use of servile fear is evil, for according to a gloss on Romans 8:15, "if a man do anything through fear, although the deed be good, it is not well done." Therefore servile fear is not good.
Objection 2. Further, no good grows from a sinful root. Now servile fear grows from a sinful root, because when commenting on Job 3:11, "Why did I not die in the womb?" Gregory says (Moral. iv, 25): "When a man dreads the punishment which confronts him for his sin and no longer loves the friendship of God which he has lost, his fear is born of pride, not of humility." Therefore servile fear is evil.
Objection 3. Further, just as mercenary love is opposed to the love of charity, so is servile fear, apparently, opposed to chaste fear. But mercenary love is always evil. Therefore servile fear is also.
On the contrary, Nothing evil is from the Holy Ghost. But servile fear is from the Holy Ghost, since a gloss on Romans 8:15, "You have not received the spirit of bondage," etc. says: "It is the one same spirit that bestows two fears, viz. servile and chaste fear." Therefore servile fear is not evil.
I answer that, It is owing to its servility that servile fear may be evil. For servitude is opposed to freedom. Since, then, "what is free is cause of itself" (Metaph. i, 2), a slave is one who does not act as cause of his own action, but as though moved from without. Now whoever does a thing through love, does it of himself so to speak, because it is by his own inclination that he is moved to act: so that it is contrary to the very notion of servility that one should act from love. Consequently servile fear as such is contrary to charity: so that if servility were essential to fear, servile fear would be evil simply, even as adultery is evil simply, because that which makes it contrary to charity belongs to its very species.
This servility, however, does not belong to the species of servile fear, even as neither does lifelessness to the species of lifeless faith. For the species of a moral habit or act is taken from the object. Now the object of servile fear is punishment, and it is by accident that, either the good to which the punishment is contrary, is loved as the last end, and that consequently the punishment is feared as the greatest evil, which is the case with one who is devoid of charity, or that the punishment is directed to God as its end, and that, consequently, it is not feared as the greatest evil, which is the case with one who has charity. For the species of a habit is not destroyed through its object or end being directed to a further end. Consequently servile fear is substantially good, but is servility is evil.
Reply to Objection 1. This saying of Augustine is to be applied to a man who does something through servile fear as such, so that he loves not justice, and fears nothing but the punishment.
Reply to Objection 2. Servile fear as to its substance is not born of pride, but its servility is, inasmuch as man is unwilling, by love, to subject his affections to the yoke of justice.
Reply to Objection 3. Mercenary love is that whereby God is loved for the sake of worldly goods, and this is, of itself, contrary to charity, so that mercenary love is always evil. But servile fear, as to its substance, implies merely fear of punishment, whether or not this be feared as the principal evil.
Source: Summa Theologica (New Advent)