One mortal sin forfeits all friendship with God, forfeits all sanctifying grace, and forfeits the ability to merit by performing good acts with the proper intentions.
Addressing whether the damned demerit, St. Thomas Aquinas writes (Summa Theologica suppl. q. 98 a. 6 c.):
We must draw a distinction between the damned before the judgment day and after. For all are agreed that after the judgment day there will be neither merit nor demerit. The reason for this is because merit or demerit is directed to the attainment of some further good or evil: and after the day of judgment good and evil will have reached their ultimate consummation, so that there will be no further addition to good or evil. Consequently, good will in the blessed will not be a merit but a reward, and evil will in the damned will be not a demerit but a punishment only. For works of virtue belong especially to the state of happiness and their contraries to the state of unhappiness (Ethic. i, 9,10).
A person who commits adultery and murders certainly deserves more of a penalty than if he only committed adultery. In both cases, these are mortal sins (due to the grave matter and assuming they're committed freely and with the full knowledge of their sinfulness), which causes the supernatural death of his soul. It's not like his soul is less supernaturally dead in the former case and more dead in the latter.
Addressing whether every act of an unbeliever is a sin, St. Thomas Aquinas answers (Summa Theologica II-II q. 10 a. 4
…mortal sin takes away sanctifying grace, but does not wholly corrupt the good of nature. Since therefore, unbelief is a mortal sin, unbelievers are without grace indeed, yet some good of nature remains in them. Consequently it is evident that unbelievers cannot do those good works which proceed from grace, viz. meritorious works; yet they can, to a certain extent, do those good works for which the good of nature suffices.
Hence it does not follow that they sin in everything they do; but whenever they do anything out of their unbelief, then they sin. For even as one who has the faith, can commit an actual sin, venial or even mortal, which he does not refer to the end of faith, so too, an unbeliever can do a good deed in a matter which he does not refer to the end of his unbelief.