@comeonnow0
After this chapter with more clear context, I take that whole bubble thought as "You already know I can hurt someone, even my close friend, that badly on purpose because that what I told you, and yet, you still continue without worrying about the possibility of me doing the samething on you. Why don't you just give up like I did? I hate people like you because you remind how weak I am for not able to fight for my friends like what you're doing right now."
So yeah, it was not a plot hole and he did not lie with his thought, it just did not have a clear context back then. His reactions entire arc had been showing that he's not evil, that there were more to it, and this chapter proves it.
And you really think when someone choose to take the blames on themselves for someone's sake, they would just easily tell others, especially people who are involved, about it? I know I wouldn't because what is the point of becoming the bad guy if you can't keep it as a secret to people close to you? Not to mention, Shinki hurt Dai physically, if he told Dai much sooner, he might hurt Dai emotionally making him thinking Shinki had abandoned him and not keeping their promise.
I agree that this is an underwhelming arc with weak writing, and ends up not contribute much to Dai as a character at all, I just don't agree with your points there.