/summa/SS/SSII/SS_Q59:
We must now consider injustice, under which head there are four points of inquiry:
(1) Whether injustice is a special vice?
(2) Whether it is proper to the unjust man to do unjust deeds?
(3) Whether one can suffer injustice willingly?
(4) Whether injustice is a mortal sin according to its genus?