Love what is admirable, but hate the sin

     "How can Dante, for example, so obviously admire his own teacher, Brunetto Latini . . . and nonetheless place him in Hell?  If Brunetto Latini was so admirable in so many ways, as indeed he was, how can he suffer that unqualified condemnation for the sin of sodomy which places him in the Inferno?  The answer is clearly given by Aquinas:  the doing of many good deeds is perfectly compatible with the perverse choosing of something in oneself which is defect and error and affirming it as what one intends unalterably to be.  And it is this choice which is one's own choice of exclusion from the community of the perfected."

     Alasdair MacIntyre, Three rival versions of moral inquiry:  encyclopaedia, genealogy, and tradition, being the Gifford Lectures delivered in the University of Edinburgh in 1988 (Notre Dame, IN:  University of Notre Dame Press, 1990), 144.  "What would it be for the sequences of the Summa . . . to be mirrored in the enacted dramatic narratives of particular human lives lived out in particular communities?  Aquinas himself does not supply an answer to this question [(except insofar as he lived his own life 'with a rare singleness of purpose, with that purity of heart, which as Kierkegaard said, is to will one thing')], but Dante does" (142-143).  On pp. 145-148, MacIntyre then gives the genealogical (or Nietzschean) deconstruction of this account, and the outlines of a tradition-al (or Thomistic) reversal and reestablishment.
     What are the sources of this in Aquinas himself?

• ST I-II.73.10 ("Whether the excellence of the person sinning aggravates the sin?"), quoting Isidore, "'A sin is deemed so much the more grievous as the sinner is held to be a more excellent person [(tanto maius cognoscitur peccatum esse, quanto maior qui peccat habetur)].'"  But the examples given here (the magistrate who offends against justice, the priest who offends against chastity) seem to indicate that the greater excellence must be in the same respect.  So I'm not sure that this is the passage to which MacIntyre refers.