Atlanna having little choice but to leave her caring lover Thomas and their son Arthur in order to protect them.
Also, poor toddler Arthur being traumatized by the attack of the Atlantean guards.
Not to mention their poor dog. He may have survived though...
Tom spends every morning for decades walking the dock like a widow's wharf, waiting for Atlanna's return, still hoping even though he's heard through his son that Atlanna is dead.
The Atlantean's response to the humans' behaviour towards the oceans is a harsh, but well-deserved retaliation in the form of a tidal wave carrying all of man's submarine wrecks and collective trash, and Orm later mentions that Atlantean children have been poisoned thanks to this sort of thing. No wonder so many ocean dwellers have so much contempt for the surface.
Young Arthur’s belief that Atlanna abandoned him and his father because she didn’t love them.
For the most part, Atlanna's disappearance (later revealed as a mandated "sacrifice"/execution by King Orvax) clearly impacted everyone that mattered to her, and nearly every one's motivation is colored by what she represents (or, more importantly), what her loss represented to them.
For Arthur, losing his mother and never knowing properly what happened to her (not helped by Vulko equivocating with him) definitely hardened his prejudice against his heritage, apart from the issues he understandably has growing up without a mother.
The Atlanteans who she raised, for that matter, don't fare any better. It's suggested both her son Orm/Ocean Master and Mera grew under her teachings, and both clearly took her disappearance quite hard: Orm, clearly torn by love of his mother and his father Orvax's values, lived with the burden of adhering to Atlantis's xenophobic values most of his life—and it definitely coloured his tenure as king. His conversation with Arthur before their first battle showed just how conflicted he was between following his father's ideals and the love for Arthur Atlanna instilled in him. Mera, in turn, clearly adores Atlanna and her teachings (especially considering her father, King Nereus, isn't exactly the warmest parent), and she clearly smarts from losing her most positive mentor in life.
When a flood overtakes their truck and they get separated, Arthur frantically searches the debris screaming for his dad, and its clear he has little hope of finding him in the chaos of the tsunami. Fortunately, Mera arrives and is able to find and save Thomas from drowning thanks to her powers.
Despite barely meeting each other, Arthur admits to Orm before their duel that he "always wanted to meet his little brother", just to show that they'd have each other in the world. Only now to see what Orm is planning, there's clearly a hint of sadness that he turned out to be a jerkass.
Orm himself isn't immune to Arthur's words. For just a moment, he's taken aback and hesitates when confronted with this confession, before going ahead with his plans.
You actually watch these two men fantasize, for a moment, about the childhood they might have had together. . . before they come back to the reality that's forcing them to try and kill each other.
As he watches Atlanna's sons fight to the death, Vulko quietly laments how he's failing her after promising to protect Arthur.
When Orm finally confronts Vulko for his betrayal, it's clear that he feels genuinely betrayed and sad by his mentor for choosing Arthur over him. Think about it. With Orvax and Atlanna dead, Vulko was perhaps the only familial figure he had through his life and the fact that his mentor chose his brother who had never cared for Atlantis over Orm himself who had devoted all his life to Atlantis obviously hurts him, his sudden shouting and clear anger when he demands to know why his teacher and vizier would betray him.
Vulko himself makes it clear he is by no means happy over having to pit Orm and Arthur against one another. Orm may have been Atlanna's second son and the one she had with King Orvax and he may have been corrupted, but he was still a student of Vulko and Vulko makes it clear that he has tried repeatedly to change Orm and strive him from the dark path that he has taken with the absence of his mother and father only to fail. His expression and tone when he muses on how he has advised Orm against it makes it clear he is sad that he failed to teach Orm better. When Orm rants on how Vulko had chosen Arthur from the beginning to take the throne from him, Vulko is clearly hurt. In his mind, he clearly did not want to have both his students fight one another and while it was Orm that forced him to do it, he does acknowledge that it was also partly his failure as a mentor that lead to the situation escalating as it is.
Made even worse by how one could tell that while Orm was visibly reluctant to fight Vulko, both because he really doesn't want to kill him and because he was wary of his mentor's fighting prowess, Vulko doesn't even try to fight back or escape without killing Orm. He just lets the guards take him to prison, despite knowing fully well Orm had no reason to spare him, all while clearly looking remorseful. The reminder of his failure to be a proper teacher and how he could have prevented all the war had he been better in raising Orm seemed to have made him resigned to his fate.
The look on Arthur's face when Orm breaks Atlanna's quindent.
Orm does some horrible things to his fellow ocean-dwellers just to get his way. He murders the Fisherman King and forces his young daughter (who had to witness the whole thing) and their army into an allegiance with him. Then he wages war on the Brine Kingdom for refusing to compromise and hacks the arm off the Brine King.
One really must feel sorry for the Fishermen Kingdom. As an enlightened culture, they believe in educating the surface world and not subjugating it. However, when their king refuses to form an alliance of conquest with the Atlanteans, Orm murders him and forces his daughter into leading her people into battle as his puppet queen. To see such an honorable and benevolent nation being forced by a vengeful tyrant into carrying out violence is just heartbreaking.
When Mera saves Arthur, she feels she may never be able to return to Xebel and that she's betrayed her land and her father. Thankfully she's wrong, as Nereus makes it clear to Orm that their political alliance works on Mera being rescued and unharmed.
They may be ruthless pirates, but there’s something genuinely sad about David desperately trying to free his father after Arthur refuses to save him. This is followed by Jesse pulling out a grenade and ordering his son to go as he prepares to die. Even Arthur eventually feels remorsefor not doing anything to prevent this, acknowledging Black Manta's resentment of him basically is his own doing.
There is a definite sadness to Arthur admitting to Mera how much he feels like a failure, given he believes he's done nothing but fail at the one thing he thought he could really contribute to in their mission (fighting).
Karathen (the monster guarding King Atlan's trident) just wanted someone to talk with. She hasn't had anyone to talk with since the death of King Atlan thousands of years ago.
Orm spends the entire movie calling out his mother as a traitor. When he sees her again, he's just so relieved...
Considering that Orm was raised by Orvax, the man who sent his wife Atlanna to be devoured by the Trench as punishment for having run away from him and giving birth to a surface dweller's bastard, it's likely that Orm had been subjected to such venomous rhetoric from an early age by his father, in addition to Orvax's general xenophobia, until he started aping that same rhetoric as his own as a coping mechanism along with the indoctrination.
The fate of King Atlan. He was so ashamed about what happened after the experiments with the trident of Altan that led to Atlantis being sunk under the sea, he exiled himself to the Hidden Sea in atonement and prevent the trident from being misused.
