Objection 1. It would seem that the souls in Purgatory are punished by the demons; for, according to the Master, "they will have for torturers in their pains, those who were their tempters in sin." Now the demons tempt us to sin, not only mortal, but also venial when they fail in the former. Therefore in Purgatory also they will torture souls on account of venial sins.
Objection 2. Further, the just are competent to be cleansed from sin both in this life and afterwards. Now, in this life, they are cleansed by pains inflicted by the devil, as was the case with Job. Therefore after this life also, those who have to be cleansed will be punished by the demons.
On the contrary, It were unjust that he who has triumphed over someone, should be subjected to him after victory. Now those who are in Purgatory have triumphed over the demons, since they died without mortal sin. Therefore they will not be subjected to them through being punished by them.
I answer that, As after the Judgment day the Divine justice will kindle the fire with which the damned will be punished for ever, even so now the elect are cleansed after this life by the Divine justice alone, and neither by the ministry of the demons whom they have vanquished, nor by the ministry of the angels who would not inflict such tortures on their fellow-citizens. It is, however, possible that they take them to the place of punishment: also that even the demons, who rejoice in the punishment of man, accompany them and stand by while they are being cleansed, both that they may be sated with their pains, and that when these leave their bodies, they may find something of their own in them. But in this life, while there is yet time for the combat, men are punished both by the wicked angels as foes, as instanced in Job, and by the good angels, as instanced in Jacob, the sinew of whose thigh shrank at the angel's touch [Genesis 32:25]. Moreover, Dionysius says explicitly that the good angels sometimes inflict punishment.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)