Thomas Aquinas

“ I answer that, The evil words of which we are speaking now are those whereby evil is uttered against someone by way of command or desire. Now to wish evil to another man, or to conduce to that evil by commanding it, is, of its very nature, contrary to charity whereby we love our neighbor by desiring his good. ”

“ But the mode of speaking is not the same, for in the case of the four vices mentioned above, evil of fault is spoken by way of assertion, whereas in the case of cursing evil of punishment is spoken, either by causing it in the form of a command, or by wishing it. Now the utterance itself of a person's fault is a sin, in as much as it inflicts an injury on one's neighbor, and it is more grievous to inflict an injury, than to wish to inflict it, other things being equal.
Hence backbiting considered in its generic aspect is a graver sin than the cursing which expresses a mere desire ”

“ Further, he that curses another would seem to wish him some evil either of fault or of punishment, since a curse appears to be a kind of imprecation. But it is not lawful to wish ill to anyone, indeed we are bound to pray that all may be delivered from evil. Therefore it is unlawful for any man to curse. ”

“ Although the sinner's sentiments cannot be perceived in themselves, they can be perceived through some manifest sin, which has to be punished. Likewise although it is not possible to know whom God curses in respect of final reprobation, it is possible to know who is accursed of God in respect of being guilty of present sin.
Art. 2 - Whether it is lawful to curse an irrational creature? Objection 1: It would seem that it is unlawful to curse an irrational creature. Cursing would seem to be lawful chiefly in its relation to punishment. ”

“ Hence in these two ways of evil speaking, by way of command and by way of desire, there is the same aspect of lawfulness and unlawfulness, for if a man commands or desires another's evil, as evil, being intent on the evil itself, then evil speaking will be unlawful in both ways, and this is what is meant by cursing. On the other hand if a man commands or desires another's evil under the aspect of good, it is lawful ”

“ Further, a gloss on Num. 23:8, "How shall I curse whom God hath not cursed?" says: "There cannot be a just cause for cursing a sinner if one be ignorant of his sentiments." Now one man cannot know another man's sentiments, nor whether he is cursed by God. Therefore no man may lawfully curse another. ”

“ Further, that which proceeds from a slight movement of the mind does not seem to be generically a mortal sin. But cursing sometimes arises from a slight movement. Therefore cursing is not a mortal sin. ”

“ Further, the devil exceeds all in malice on account of his obstinacy. But it is not lawful to curse the devil, as neither is it lawful to curse oneself; for it is written (Ecclus. 21:30) : "While the ungodly curseth the devil, he curseth his own soul." Much less therefore is it lawful to curse a man. ”