I really can only view Nea's story as self sacrifice.
Nea realized his fate was to become the Earl and destroy the world. He didn't have any options, because he wasn't just a Noah, he was the Earl.
So he made a new option.
If him being a Noah and the Earl would lead to the world being destroyed, then all he had to do was destroy that first. He (begrudgingly) accepted help from Allen and Cross because he realized he couldn't do it all on his own, especially when it came to the ark.
And then there was Mana.
He did all of it—killing the other Noah, working with Cross & Allen, even knowing he was the Earl...—he did it all for Mana.
This is where we get into the theory section.
I can't help but feel that he knew they were both the Earl. I feel like sometime after he awakened as a Noah, Nea began regaining Adam's memories- not his Noah memories, but Adam's memories of being at the Campbell manor and such. He realized him and Mana weren't twins and that he himself wasn't just a Noah, he was the Noah. And with that, an inkling that Mana was the same.
And yet, despite knowing that they weren't actually brothers, that if he wanted to prevent the Earl—himself—from destroying the world he would have to die and most likely kill his beloved brother as well—he still stayed with him. He never left Mana's side and Mana never left his.
...So much so that those visuals are still imprinted on Earl!Mana's memories.
Nea's fate was to destroy the world as the Earl. But just like how your path isn't set until you make your own footprints in the ground, Nea forged his own path. A hard, lonely path that involved becoming a murderer and leading to his own murder-suicide.
I really feel like that was Nea's plan- kill the Noah, kill Mana, and then kill himself once he became the only living fragment of the Earl. After all, this is his last journey.
Nea decided to kill Mana after killing the Noah, but Road stopped him before he crossed that line. I really feel like that's what was happening in the flashback of Road holding Mana.
Nea knew he and Mana were both the Earl and that's what led him to killing the other Noah and eventually trying to kill Mana.
It's all a self-sacrifice but it's all a sacrifice for Mana's sake. Nea was the one to decide to become a murderer, to decide to kill Mana before he becomes the Earl. And now because of this path he's chosen, he's alone.
Nea is quick to shut out any outside help now that he realizes he is truly alone- Mana is still mostly consumed by the Earl, Cross is "dead", and Allen has no memory of him. Allen was alone when he fled the order but he still had friends searching for him and eventually helping him. Nea has none of that now. But that's the life he chose.
It's both a self-sacrifice in the physical sense, that he plans on killing the last fragment of the Earl (himself) and a more mental sense as he takes on the burden of the world onto himself and refuses to let anyone else help.
Just like Allen and Mana have developed "masks" to hide their pain from others, Nea has one to shield his pain as well. It would be so easy for him to try to help Allen regain his memories, but instead he has to keep up the tough guy act and pretend he doesn't care.
He has tried to throw away everything including his emotions with his self-sacrificial journey but at the end of the day he is still a human trying to mask his pain because he knows the path he's chosen is nothing but tragedy.
The only path left for Nea now is to kill Mana (and probably himself) and Nea is determined to not let anyone get close to him again as he sets out on his path of destruction again.
It's so wildly self-destructive but also such an interesting foil to Allen's self-sacrificial tendencies—Allen throws himself in danger to protect others from the pain he's experienced all while pretending to be someone he's not, but Nea never had the luxury of choosing to protect people like that. The best way Nea can protect the people he loves is if he's dead.
There is no doubt in my mind that Nea (still) loves Mana and Mana (still) loves Nea (and even further, that the pieces of Earl!Mana that are so attached to Nea are pieces of Mana that are still alive).
I really see Nea's whole story as being an entirely self-sacrificial path to try to save his beloved brother from becoming a monster and shoving the entire weight of the world ending on his own shoulders, not letting others close enough to take off the burden. Except Allen.
Allen somehow got close enough to Nea to not only become the person he trusted with the ark, the Earl's prized device for fighting and creating akuma, but to take some of that burden off of Nea's shoulders. Just like we're seeing Allen's mask fall off as he's surrounded by allies again, I hope we'll see the same with Nea as he realizes he can trust Allen once more—one final time for one final journey.
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yeah that was some rambling / my own theory on Nea's goals and why he acts That Way. at the end of the day he's a pissy catboy who really wants attention from the people he loves most but can't because he's also trying to kill them </3
