Yeah I also don't understand what made ser Criston act this way. Maybe that the guy knew his secret?
Laenor's lover, Joffrey, correctly inferred that Criston was (one of) Rhaenyra's paramour(s). The guy was happy with this arrangement, probably because he and Laenor could never have had a publicly accepted relationship anyway, and fatally assumed that Criston would also be happy with it. I think basically, 1) Criston was already in a bit of a state because of guilt over breaking his oath 2) he was probably anxious about Alicent telling the King about his affair with Rhaenyra and consequently being tortured and executed (he begged Alicent not to geld him and just execute him quickly - Alicent didn't outright confirm she wouldn't execute him) 3) he is also in a state because he did all this to basically get sideman privileges, which isn't what he wants at all. having someone basically know and celebrate it like it's a good thing enraged him and sent him into full murderous bastard mode
It's also possible he is homophobic, given the world seems homophobic, but can't say for sure - if he is, he would've perceived Joffrey's comment as an even bigger slight
I think it echoes GOT in that the inciting incident to the conflict was inappropriate 'coupling' being used to destabilise centres of power (Cersei and Jaime, Joffrey Baratheon's illegitimacy, and Littlefinger pulling the strings) and cause civil war. Combined with the cultural resistance to having a woman in as the ruling monarch, Daemon has essentially kicked this into action by getting off with Rhaenyra and allowing that to get back to Otto and Viserys. He's taking a big risk here as he's planted the seeds for everyone to turn against the Targaryens - he got rid of Otto but in doing so, he's set Alicent against the Targaryens, and Alicent has children with a claim to the throne. Him killing his wife to free up space for a Targaryen wife demonstrates his confidence that nobody stands a chance against him - he doesn't give a
duck that the Royces/the Vale have substantial reason to turn on him and is certain he'll get Runestone. Powerful people saw and seemingly believed the Royce cousin's accusation of Daemon so they could turn on him as well - esp. the new hand because his son has already expressed allegiance with Alicent. Criston is now gonna side with Alicent because she's shown him mercy and compassion, but the only reason this situation happened to begin with is that she brought him in to question him about Daemon and Rhaenyra. Daemon has basically kicked off a war because he feels disenfranchised and is power-hungry :/
Sidenote RIP short-lived queen Rhea Royce - loved it when she told Daemon to go
duck the sheep if they're so preferable hahaha