It had been a rather pretentious request in the first place, Severus thought, to ask for time in the Headmaster's personal office to catch up with some famous relative that Trimble younger had (judging by the death date on his great-grandfather's frame) likely never met. It was nothing he felt obligated to respect, not at all; but one didn't like to slight portraits, who had the ability to make one's working space highly unpleasant. If Trimble was going to insist upon intentionally disrespecting him, however, it would become a necessary risk. Considering that they had hardly met at all since the man had been a passably above-mediocre N.E.W.T. student, and not a particularly pleasant one at that, Severus thought it took quite a bit of nerve to go about acting like he ought to be able to speak to everyone just as he wished.
Trimble elder was one of the few portraits he actually enjoyed speaking with, unfortunately. One of the scarce benefits Severus had realized from being a spy was that everyone was now perfectly aware he that had been a Death Eater for the better part of twenty years, and as such it was more or less expected that he should have a deep interest in the Dark Arts. His extensive library on the subject mingled freely with the other books that took up the vast majority of available wall space in his office, respectable and sinister alike, and Headmaster Trimble's opinions were frequently more reasonable than those of, say, Albus.
But that didn't mean he had to like his progeny, did it.