Iroh's main change is that he doesn't believe in fighting nearly as much and tries to avoid confrontation, he has a couple of exceptions like with Zhao but those are extreme circumstances. Iroh wasn't really a crazy lunatic he was just a general following orders to bring honor to his homeland and he believed in the lie of "sharing the fire nation's glory", Ozai and Azula might be insane, but Iroh to me never came off as a really crazy person ever. His son's death made him value life more and thus retire from his career are a leader that brings the death to hundreds of sons just like his and is why he doesn't take life so recklessly anymore. In addition Iroh purposefully kept the secret of the dragon's safe when he was a general still I believe, as he was the Dragon of The West back then, so he always was a mostly good or well meaning person he was just taking order from bad people as that is what he is supposed to do by his nation's teachings. Also we never see Iroh change in real time if we're being honest. We see him briefly as General Iroh Dragon of the West in some flashbacks, then we see him as Uncle Iroh for the rest of the series. We know he mourns and is deeply upset by the loss of his son, but we don't know what exactly happened or how long this process took.
We actually get a glimpse of Iroh’s old personality as he was taking Ba Sing Se (sorry for incorrect spelling). In it he laughed about burning it to the ground. He is a decently light hearted character, so one could interpret it as just a goof, but it could also be seen as a general lack of respect and care for another culture. After his son’s death, we see Iron take a 180 on this as he begins to see the importance of all elements and learns from them instead of destroying.
Dimitri is a person who wholly believes for a good portion of the game in executing atrocious violence to deliver justice and more importantly is convinced of this through a misunderstanding (I assume) of his loved one's last wishes.
I would like to challenge this a little. In the first battle against the bandits, Dimitri seems to have a hard time killing and says something along the lines of “this part doesn’t get any easier.” His support with Ingrid also has him try to tell her that war was vile and no system should reward the graphic deaths he saw on display in Duscar. There was also a line in game that comes after a Byleth conversation with Ashe, in it Ashe asks Byleth if Lonato really had to die, in which Byleth has to say some form of yes. Right afterwards, Dimitri barges in saying “Are you Insane!! Those weren’t just soldiers, but fathers and sons. We had no right to take their lives.” There are also multiple cutscenes where Dimitri questions the church’s ethics on killing.
In the earlier game, the only information we are given about the “boar” comes from Felix, as Dimitri shows no signs till Remire.
He has to kill everyone related to their deaths in order to ensure their voices are at peace, because he must shoulder their anguish in his mind.
Once again disagree, at least for earlier game. In the beginning his goal seems to be that of ending prejudice against Duscar and the people as he knows they are innocent. It does not seem like he had voices in his head at that period of time.
To your point though, I must acknowledge that he deemed his reasoning for going to Garret Mach as “revenge.” What that entails is sort of up in the air, though it is highly likely to be killing them. The only thing with this scene is that it came after Remire, which was the tipping point.
When Zuko tries to change in book 2 and he seems to wholly believe in that idea, and then in a lapse of judgement when temptation strikes he goes back to his old ways for a long while which ultimately causes a chain of events that he later regrets.
Putting this point here as it fits better.
I think the issue with Dimitri is that we start on book 2. Dimitri’s story is that of one who was starting to go clean and then relapsed due to events, only to climb his way out again. We are told of the boar in the past by Felix, but that is not the Dimitri we meet in Garreg Mach. This one is the going clean Zuko of book 2. The temptation for Dimitri was Remire.
As for why Remire, it is due to a likely PTSD episode that got him unhinged. Think about it. TWSITD were the cause of Duscar and Dimitri knows it. Duscar, according to Dedue, was also burnt to the ground, or put to the torch. With Ingrid, we find that Dimitri is uneasy when it comes to the dead and the brutal slaughtering. Now look at Remire, a village that TWSITD decided to light on fire and send dead bodies to slaughter the innocent. This is like if someone resurrected Batman’s parents so they can hold a gun up to another little boy’s family. What they did is sick and would definitely play to every nerve Dimitri has, explaining his extreme lashing out in this instance. After that he is unhinged but still able to keep some level of composure. The breaking point was having his once crush be the face of every horror in his life. He already likely had a ptsd episode, now imagine having a love, a literal family member, now a leader of the people he hated.
I think he is genuine in part 1. I really believe that he sees killing as wrong, but he reverted back because, well, how could he not? They literally pinpointed everything to make him tick and relapse.
Getting back to the point, I think his fix at the end makes sense. The issue was just the lack of a book one to really hammer the progression of where we start with him. He was always a good guy, but was destined to fall to some capacity with everything going on in his life. Really, I think that is the issue. Him giving up killing seems reasonable, we just lack the set up, so it seems like Garreg Mach Dimitri was just a hidden monster, when he was a Zuko type, improving by the minute.
It might be unfair to compare Dimitri one of 3 lords in a game that is quite frankly rushed in every way to Zuko a character with dozens of TV episodes worth of content (a large portion of said episodes dedicate the entirety or a significant portion to his development) to develop his evolution, but Dimitri is one of many examples of why to me this whole multi route Fire Emblem is a failure of a concept that IS lacks the full means to execute properly (not that it is easy to do so) and we need to just drop it. Dimitri is just probably one of my least favorite examples of this because I was really into his character up until the route was over and I thought about it more, but his change was so forced that it ruined the entire route for me.
The new mechanics such as a 3D roam-able Monastery also took time out of development. So they had tons of new mechanics and four routes, the game was going to be lacking in some area.
I do not mind multiple routes, but this game should have cut one or two. The church route feels the most tacked on and pointless. It was likely only put there so players do not have to go against the church when the game directs them into choosing Edelgard first. It was pointless and took up to much time. If they had to cut a second, I would say Deer. They are a better route than Edelgard, but they feel like they have no bearing on the overall narrative, plus a Dimitri vs Edelgard story could then be made to be so much more compelling.
Still mad that they pulled an amnesia with Edelgard, really robbed her of a good climax when fighting Dimitri as it was her vs a nobody, not a man she based her goals on.