i also. something about people saying primus is just incapable of caring about anyone hasn't sat right with me since any of the occtis flashbacks, and i think this episode fleshed that out quite a lot
primus tachonis believes he's capable of love, i'm sure of that. whether what he has for people is actually love is a debatable point, but like. i believe him when he said to occtis "i hope you find some degree of effortlessness one day". i believe he wasn't doing it for show when he kissed his daughters' heads before leaving with them. when he suggested to 8yo occtis he try practicing his sorcery here, and that time was of the essence, i believe that he thought of that as a kind (or at least earnest) gesture. it wasn't solely a lie to get occtis out of the room, it was a father who had a lot of expectations for his child but did want to see that child succeed in them. and a father involved enough in his children's business to distinguish their individual signet rings from each other (a task that couldn't have been obvious bc no other sundered house member could do so)
there were a lot of lies thrown around in this meeting, but i don't think him saying "he had already betrayed the family, i'm not a monster" in response to yanessa privately asking if he really killed his son just for power was one of them (not completely, anyway)
the problem is primus' rigid beliefs in societal roles and what makes someone worthy of them, and enforcing that incredibly strictly because the concept of not doing so genuinely disgusts him. yanessa also asked him if he really didn't see the benefit in having commoners on your side and he couldn't even entertain that. primus was born into nobility, so obviously he was meant to rule over the common folk, or failing that, destroy them. house royce were betraying their station, unable to keep proving their own worth (and in the minds of the other houses, leeching off their alliance), so they had to die. he had no vendetta against davinos, but their deaths were a necessity, and as someone who outranks their entire house primus was entirely within his rights to take their lives
occtis is an embarrassment, being handed such a high place in society (by primus himself) and unable to prove he belongs there. the thing that makes primus innately great is missing in him, and his attempts to prove he still has worth despite that are just a source of boredom or frustration to his father. but primus did acknowledge in episode 3 that occtis achieved what he set out to at the penteveral. there possibly is a world where, had occtis not betrayed the family, and the deva vindicta plan not required his heart, he could have gotten into good standing with his family by mastering that magic. and even if there wasn't, as long as occtis stayed loyal primus would have probably seen his death as a necessary evil. not something he enjoyed doing, or thought he was right to do, but something he was willing to do for the greater good
(when i said not completely earlier, i think the lie he told to yanessa was in avoiding the topic of tertia)
but when occtis tried to save thjazi, it doesn't matter that the attempt was a failure, bc the crime was breaking the social order. primus tachonis, by his own beliefs, is in the right to punish someone so much lower than him who had the audacity to try and foil his plans. occtis choosing to align with thjazi over his own family is breaking every principle primus holds so dear. so if there was any part of him that still cared about occtis by that point, i don't think he could physically bring himself to keep a hold of it
he may be one of the most powerful people on the planet, but he is still just a human being. and as ever i think it's far more interesting to be dealing with a man who can do horrendous things to the people he loves if he's got an internal justification for it, than dealing with a man who doesn't love at all