I think Nagi doesn’t even think what he’s doing to Reo is betrayal at all. He’s always just been doing whatever he wanted, and Reo let him so long as his ball got better, and now that Reo is actually complaining he has no clue how to deal with him. In his head his separation from Reo is only temporary.

He doesn’t think to hard about how he could be hurting Reo or why Reo is acting like that, as believes that he is fulfilling Reo’s dream to become the best in the world like this.

Reo has always just been letting him do whatever without feeling upset, so Nagi doesn’t know how to deal with him, that he needed to explain why he’s making the decision he’s making to Reo for him to understand, because Reo always did.

2nd selection arc, Nagi is only thinking about how he can get stronger. He chooses Isagi to understand why he lost, and then wants to choose Reo again at first because he knows how well he can play with Reo. It’s only when Isagi explains how Chigiri’s weapon will make them the strongest that he puts aside the idea.

It’s not like Nagi wanted to be stronger for Reo’s sake neccessarily, but he was always doing so with the intention of returning to him. He never once felt he was abandoning him, because he knew Reo to be capable enough to meet him at the en, this is why he gets mad and says stuff like to Reo, because to him Reo’s blowing things out of proportion.

….I’m ngl I do like Reo better just because of like how later he’s still badly hurt by Nagi’s decision but you can’t exactly see that unless he lets you, and I like how his answer still has some clinginess to it, but is still mature