It was in the beginning. Humanity is trapped by giant monsters, boy wants to kill all the monsters, boy gets monster powers, boy joins the army and there ya go.
Why Isayama felt the need to make it any more complicated than that, I'll never know.
Game of Thrones came out. He got bored with horror and decided he wanted to do political shit instead and turned it into his WWII revenge fantasy.
I don't know, but Eren seems to have it down pretty well.
In all seriousness, all you'd have to do mitigate this would be to just have someone bring up how shitty he was treated. If someone at least mentioned how everyone, hell, not even everyone, just mentioned how at least a few people treated him poorly, or just how he was traumatized like crazy, and was literally born insane and how no one tried to help him, Eren would feel less like such a fucking edgelord brat.
You wouldn't even have to mention how the SC was practically almost designed to make him worse. Literally all you'd have to do would be to just acknowledge how shitty Eren's life, or even just how Eren's literally always been insane, (something that characters did use to do, but then stopped for some reason), and acknowledge how they fucked up by not helping him, and only making him worse and Eren becomes a lot better character.
If just one person, even if that person was Eren himself, acknowledged how badly he was treated, and how he fucked up from the start, Eren's character would become a lot more tolerable. It still wouldn't make me sympathize, or agree with his plan, but it would make him feel a lot less like an edgy, whiny, pretentious, twat.
It's that simple.
This is going to be long. But here's how you can (sort of) justify Eren's plan. It has to be long, because genocide is inexcusable. So you have to corner him.
Show how abusive everyone was to Eren. Enforce that really well. That the scout squad he idolized was nothing more than a bunch of burnouts, sociopaths and crazy people obsessed with power. They abuse him to no end. Threaten him with death. They alienate him from his friends, who they threaten if they get too close to him. They become obsessed with Eren as a living weapon. They give him little to no freedom. Erwin is a bit darker and wants to use Levi to constantly put the pressure of death on Eren. This was when he was just a kid.
His idealism is slowly chipped away. He knows that he's dangerous and time and time again he goes out of his way to prove he isn't, but he never gets any recognition. The Scouts eventually catch his friends trying to sympathize with him and rescue him, but then they're separated into completely different units. Mikasa is imprisoned, and they lie to Eren that she's disgusted by him and never wants to see him again. He never gets to reveal his true feelings. The entirety of his idealistic persona is chipped away time and time again, the Scouts only view him as a means to an end. Hange treats him like less than human, an experimental test subject to be poked and prodded at. Erwin is cold and distant, unavailable. Levi is OCD and constantly beats and threatens Eren. Sasha is the only one nice to him, but the Scouts are on a dangerous mission, and Sasha dies due to their incompetence and uncaring nature.
He meets Zeke and feels he has a brother for the first time, but Zeke is just like the Scouts. He wants to use him as a means to an end. He manipulates Eren, and that connection to his family is shattered. The only people who ever treat him with respect are the civilian Eldians. He encounters mercy and care from them, because they feel that their situation is completely hopeless. Make things in the walls worse, so that even if Eren is a titan shifter, the people look up to him and trust him. Historia cares for him, but she's whisked away as well, forced into political duties and barred from seeing Eren. Armin is too busy trying to reform the Scouts that he neglects Eren and constantly promises him things will get better, but they never do.
When they're in Marley, he's troubled by their inhumanity to the people he cares about. The civilians of Eldia, the only ones who treated him with any sort of decency. The internment and concentration camps are the straw that breaks the camel's back. He just snaps. He doesn't care about the goals of the scouts anymore. He finds out Zeke is going to sterilize the people he cares about. He doesn't care about the scouts or the other organizations. They are too corrupt for him and he doesn't care what their plans are. He feels that the imprisonment by Marley is the cause for the Scouts desperation and carelessness. He feels little to no companionship with them. Eventually, they eliminate all the titans on the island, develop titan counters and steal tech to be militarily equal to Marley thanks to their secret missions. They surround Paradis Island with cannons and anti-air weapons, making it almost impossible to siege. The loss of the titans, the huge walled city and the massive amount of firepower Paradis possesses brings them to the table. The plan is to negotiate with Marley. But the only thing the higher-ups care about is Marley to leave them alone. They don't care about people over there in the camps. They don't care what terror Marley wreaked for hundreds of years on innocent people. This causes Eren to snap. He has been betrayed so many times, that he simply sees no other option. He feels that there's no way Marley will abide by anything they say and the fact that the military is willing to just leave people in the camps is the ultimate betrayal. Eren himself sneaks into meetings, spies on Marley and he finds out they're just going to bide their time and wipe Paradis off the map. That the military and nobles just want to preserve their own lifestyle. He's driven into a corner by the callousness of the scouts, the destruction of his idealism, the lack of humanity from the nobility and military, the death and imprisonment of the people who care about him and Armin's complete failure to reform the Scouts.
He sees no other way to deal with things, because he's never been offered one. The people in power treat him like a nuke that must be executed if out of control, and treat him as less than human. He's seen everyone he cares about disappear or die. He sees the civilians suffering under the thumb of the military, who now he views as traitors for trying to make a pact with Marley now that they have Eren. He refuses to go along with this. His suffering reaches its culmination when he finds out that Mikasa never left him, but was purposefully kept away from him. Maybe even sent over to the mainland, back to 'her people', as far from Eren as possible. Maybe some sort of trade for 'peace'. It utterly breaks him and he no longer gives a shit. Before he can do anything, the scouts dismember him and lock him up, suspecting him of something. They keep him deep underground. It looks hopeless, until Zeke comes to rescue him. Because he's family and Eren has pretended that he would go along with his plan. Its here that Eren betrays Zeke, using his royal blood to get into PATHs.
He meets Ymir in PATHs. Ymir is much like Eren. She was used and abused for her abilities. Kept as a weapon, kept from people she cared about. Tortured. Murdered. Eaten alive countless times, so she split apart into all the different titan shifters. She resonates with Eren's pain. That's how Eren gets Ymir to follow him. Zeke is then ripped apart by Eren and Ymir, sending his shattered soul throughout space and time, forever.
This is when Eren activates the rumbling and then goes on a rampage. There's nothing left for him. He has no former comrades. His only loyalty is to the people in the camps and the civilians on the island. He will burn Marley down to find Mikasa. Then he basically just fucking kills all the Scouts, sparing Armin. He massacres the nobility, and everything on Marley. The only people spared are the Eldians that live there.
I know this is super long, but you have to have a lot of justification for what Eren does. As of now, there really is none. He's just a madman. In this scenario, he's an abused kid who has every part of him crushed, the humanity beaten out of him and everyone he cares about dies or disappears. He exists only in this harsh world of callousness where his people are persecuted and betrayed by those sworn to protect them. It is the only option.
tl;dr: The scouts are much more callous and evil, Eren is abused constantly, and they want to make peace with Marley for their own selfish goals. The only people who cared about Eren were the civilians. He fights for them and the people in the camps.
