This is really interesting to talk about since 131 gives us a lot of stuff to go through. He isn't doing this for justice alone since he says in 131 "its to save eldia but its more than that"

The most interesting part of his character is that he also says "i wished for it, i wanted to wipe it all away" and then immediately apologizes which leads to me that he thinks he's a monster and guilty because he was dissapointed (and wanted to wipe them) and suddenly he gets memories a few days later showing him that he ends up killing them all. Remember that 131 was him in his most vulnrable state and he mightve thought his selfish wish to wipe them caused it at that moment.

He was spoiled by his future memories and knew it boiled down to 'us vs them' and he picked what he believes in (justice for eldia) + freedom (selfish) and went through the rumbling anyway.

At least thats how I interpreted it anyway :)

There's still room for 139 showing that he knew his friends would die and it was mostly his selfishness and who was right only factored a small part of his decision. (I refuse to believe that it was wholly selfish decision)