
It's reeeeally sad, but I think Eren truly believes in what he's said. He was an extremist and a idealist from the start, and now that he is an adult his obsession with freedom has worsened, and he doesn't have one direct enemy now, so he's not focused only on one thing like before and has started to pursue a bigger goal. I think he believes he's saving the world in his own way (or Zeke's).
Also, he has so little time left he doesn't care about friends anymore, he only wants to do something for good before dying. He wants to die fighting.

Does this "how could I decide to do sth so cruel" mean yangyang wants to reject him?

I agree with Lolatako above. I also think YuYang doesn't want LiHuan to go through the "burden" and troubles of being a homosexual in this story, since YuYang had to and still has to experience all these troubles himself. He probably is trying to be kind to not let LiHuan deal with this, but he's also a bit selfish in the way that he just doesn't want to take LiHuan seriously because of personal reasons (maybe like it's too good to be true that someone actually likes him, insecurity issues, or whatever else). Let's just wait for LiHuan to win him more. Have faith in our big guy!
Uuuuu this looks like some more plot, doesn’t it? I hope the author continues the story a little bit more because it’s sooo good