Experience Tumblr Like Never Before
This realization does happen early enough between the second and third book and thus gives us time to watch Damen react to his own changing perspective as well.
You can watch when Damen witnesses the scheming that Laurent's had to put up with, being framed for attacking a village, prompting retaliation, and even his efforts to find the real culprits being turned against him too. Damen is still a prince; he's not naive enough to not recognize that Laurent is facing opposition from almost every direction and has been forced to become a stone-cold bitch because everyone he's ever been kind to or relied upon has been threatened, killed, or systematically turned against him.
Damen sees the horrors of the border, where the people in Delpha are still Veretian at heart, no matter what someone drawing borders on a map says. He is heartbroken to see his own people happy to slaughter innocents just because of the feud between the kingdoms. Damen doesn't just become forced to rethink Laurent, he's forced to rethink Vere as a whole as well as his own ignorance of the things Laurent has been embroiled within for years now.
Damen is smart enough to comprehend what it all means, and he's strong enough to go through the existential crisis of admitting to himself that maybe he, and the way his father raised him, were WRONG. Damen is strong enough to let go of his pride and LEARN, and that's what makes him a worthy prince - as well as someone who is capable of falling in love with Laurent and having Laurent fall in love with him, despite Laurent's best efforts to hate him.
~Rant incoming as always~
And because I'm a Laurent lover myself:
When they are forced to get along in Prince's Gambit, you can see all the moments Laurent is shocked how hard Damen fights for him, how he doesn't escape or betray Laurent the moment he has the chance, and how Laurent really is weak to not just loyalty but competence. Damen absolutely can defeat him through sheer strength, even though Laurent has spent the last six years trying to prepare himself to kill Damen, and he doubts his own ability to outmaneuver Damen in a fight because he's blinded by his own inferiority complex that the Regent has instilled into him by force and that Laurent has to systematically unlearn.
Now, Laurent has to come to terms with the fact that if Damen's loyalties turn against him as well, Laurent's heart might not be able to take it either. Laurent is literally vulnerable to Damen in every way imaginable, and he's pissed. He covers it through sarcasm and banter, like when Damen admits he could grab Laurent and turn him over to Makedon's passing troops, but then is honestly relieved when Damen DOESN'T BETRAY HIM, AGAIN. Damen actually kills one of his own people by throwing a sword in a completely irrational maneuver, and you know Laurent is going through shit when you consider that he must think Damen only supports him because Laurent is just better than the Regent, the lesser of two evils, the spare prince that's only worthy because Auguste is dead...but maybe Damen also is just that good of a person.
Remember that Laurent isn't fooled by Damen's "undercover" identity for a second, so he's seething at the idea that Damen is the only one he can be honest with, if only because it's in Damen's best interests to not betray Laurent. He's coping with the idea that he and Damen would have absolutely gotten along if there wasn't this massive gap of them being from opposing kingdoms but also the matter of Auguste.
Once Damen's identity gets exposed and Laurent is like "Yes, I know, asshole, you're not exactly subtle", Laurent becomes a defensive bitch again for the first half of Kings Rising because the two of them really do have to confront that Damen killed Auguste and incidentally ruined Laurent's life. It wasn't personal, Damen had no idea any of that would lead to the other things - he had no control over the Regent's actions, and killing Auguste was just killing the enemy in wars they didn't start and didn't have the option to just sit down and talk about.
Damen trying to say, "He died quickly," and Laurent's immediate defensive reply, "Like gutting a pig?" OOOOOFF FUCK LAURENT THAT WAS MY HEART
Damen and Laurent beating the shit out of each other as Laurent tries to kill him, but he has to yield and admit he would have died if Damen wanted him dead, but Laurent saying he'd rather have just died never getting to know Damen as a person because Auguste was everything to him and dying would be easier than seeing how he and Damen could have gotten along only to be denied it. Then Damen ending the confrontation with, "I wish..." AND HE CAN'T FINISH BECAUSE HE KNOWS WISHING WON'T CHANGE ANYTHING
Damen KNOWS he ruined Laurent's life by killing Auguste but he also knows it wasn't personal to him, but it became MASSIVELY personal to Laurent. He regrets it, he knows he regrets it, but regretting won't bring Auguste or Laurent's childhood back.
The cherry on top comes as Laurent fights Kastor and Damen realizes that Laurent absolutely IS AND WAS skilled enough to beat Damen in a fight, he was just being held back by his own emotions (and maybe a knife wound to the shoulder) making him desperate and sloppy. Laurent killing Kastor essentially makes them even as they each took a brother from one another; on the one hand their fates were *necessary* to make Damen and Laurent who they are today, but on the other hand - at what cost?
Laurent being told by Damen that he's a worthy prince by the one person he thinks he can't overcome, in contrast to the Regent telling him he isn't worthy and trying to force him to admit that he can't overcome his uncle - when in reality Laurent IS able to overcome them both.
Ugh, the extras when we finally get to see Laurent acting like a young man who can let his walls down and grieve, who can mess around with flowers and put himself beneath someone he loves without fear, who can just start throwing olives into a barfight for the miniscule layer of chaos. I love him. Damen loves him.
Anyway so I have a fanfic that's half complete where I ramble like this throughout:
https://archiveofourown.org/works/52964602/chapters/133982485
Thinking about how Damen does not even begin to comprehend the absolute life-altering trauma he caused Laurent by killing Auguste until like halfway through Prince's Gambit. Thinking about how their mutual dehumanization of each other led Damen to see Laurent as incapable of love or affection for anyone, he never even considers that Laurent loved his brother and was shattered by his death, never shows a shred of sympathy, his first assumption was that Laurent resented Auguste for being the golden child/crowned prince, and it's only when Paschal looks at him like he's crazy and says "no, he loved him." that he begins to realize the Laurent he's been experiencing is one that in many ways *he helped create* and that the purest form of Laurent was a sweet, shy little boy who loved his brother without a cruel bone in his body, he never wanted power or glory or anything, all he wanted was his big brother, and Damen killed that version of Laurent when he killed Auguste.
I think that is in part how Damen begins to come to forgive Laurent, or at the very least to begin to sympathize with him, realizing that in a fucked up kind of way, everything Laurent does to him, while still totally being first and foremost Laurent's responsibility and moral failures to atone for, is partially a consequence of his own actions, that he helped turn Laurent into the tangled ball of pulsating yearning in the shape of a man that he is.
I think realizing how wrong he'd been about the kind of man Laurent was, was what began his journey to coming to terms with the kind of man he, Damen, was at the beginning of the story. When he first meets Laurent he thinks he has him pinned and describes him as arrogant, self-absorbed, self-serving, spoilt, and "raised to overestimate his own worth", which in hindsight is definitely meant to be projection because those are all ways that Damen himself could be described at the beginning of the story, something he basically admits to at the end of Kings Rising when he reflects on the version of himself that existed before he was imprisoned.