Objection 1. It would seem that guile is not a sin pertaining to craftiness. For sin, especially mortal, has no place in perfect men. Yet a certain guile is to be found in them, according to 2 Corinthians 12:16, "Being crafty I caught you by guile." Therefore guile is not always a sin.
Objection 2. Further, guile seems to pertain chiefly to the tongue, according to Psalm 5:11, "They dealt deceitfully with their tongues." Now craftiness like prudence is in the very act of reason. Therefore guile does not pertain to craftiness.
Objection 3. Further, it is written (Proverbs 12:20): "Guile [Douay: 'Deceit'] is in the heart of them that think evil things." But the thought of evil things does not always pertain to craftiness. Therefore guile does not seem to belong to craftiness.
On the contrary, Craftiness aims at lying in wait, according to Ephesians 4:14, "By cunning craftiness by which they lie in wait to deceive": and guile aims at this also. Therefore guile pertains to craftiness.
I answer that, As stated above (Article 3), it belongs to craftiness to adopt ways that are not true but counterfeit and apparently true, in order to attain some end either good or evil. Now the adopting of such ways may be subjected to a twofold consideration; first, as regards the process of thinking them out, and this belongs properly to craftiness, even as thinking out right ways to a due end belongs to prudence. Secondly the adopting of such like ways may be considered with regard to their actual execution, and in this way it belongs to guile. Hence guile denotes a certain execution of craftiness, and accordingly belongs thereto.
Reply to Objection 1. Just as craftiness is taken properly in a bad sense, and improperly in a good sense, so too is guile which is the execution of craftiness.
Reply to Objection 2. The execution of craftiness with the purpose of deceiving, is effected first and foremost by words, which hold the chief place among those signs whereby a man signifies something to another man, as Augustine states (De Doctr. Christ. ii, 3), hence guile is ascribed chiefly to speech. Yet guile may happen also in deeds, according to Psalm 104:25, "And to deal deceitfully with his servants." Guile is also in the heart, according to Sirach 19:23, "His interior is full of deceit," but this is to devise deceits, according to Psalm 37:13: "They studied deceits all the day long."
Reply to Objection 3. Whoever purposes to do some evil deed, must needs devise certain ways of attaining his purpose, and for the most part he devises deceitful ways, whereby the more easily to obtain his end. Nevertheless it happens sometimes that evil is done openly and by violence without craftiness and guile; but as this is more difficult, it is of less frequent occurrence.
Source: Summa Theologica (New Advent)