
Why are you reading yaoi then? Most yaoi are cringe

If this was a hero or even a regular person I guess I could agree with you. But in his case it was the first time IN YEARS of abuse where he dared to fight back. It felt to me more like an attempt to set himself free from the hold his boss had on him than a random victory speech. Specially considering that up to that point there was no evidence that he would survive that confrontation.
I guess talking things out with Rui gave him back a little bit of the confidence he had lost during all those horrible years. But he still doesn't feel worthy enough to "impose" on others by asking for help and he didn't want to put Rui in danger. So he went on his own to confront his abuser, even though he had 99,9999...% of certainty that he wasn't gonna walk out of there alive. But it was something he felt he had to do.
Are random speeches about how bad guys never win and how love/friendship conquers it all cringy and corny? Yes. Does the bad guy cares? No. But who cares? If you're going to die anyway, might as well say whatever you want to say. Will the bad guys look back and laugh at how lame you were as they get rid of your body? Probably. But if it made you feel a little better before you died, then isn't it fine?
In Natori's case - and in most protagonist's cases, let's be honest - he didn't actually die. We can't kill our main character that easily, right? Will he regret that speech? No, certainly not. Unlike the usual airheaded MC, Natori wasn't making a speech about his beliefs or his goals. I dare to say it was his first step towards freedom. I don't know if you ever have been in an abusive relationship of any kind (I hope not), but let me tell you this: finally having the strength to talk back to your aggressor, and doing that while aware that you are right, feels really fucking good. And it's a necessary step towards healing. In therapy, in cases where you can't confront your aggressor for any reason, they usually tell you to write a letter or to do a little role-play as if you were. Because putting those thoughts out there is an important part of processing the shit you have been through.
So long story short, because of Natori's back story I can't agree with you that that speech was unnecessary.
Am i the only one that feels a little embarassed when the good characters begin to make an emotional long ass speech for why they're doing what they're doing in front of the villan of the story? 'Cause sometimes i feel like, they're bad,they're assholes, they don't care that you fell in love or whatever so they're like "why are you telling me all this bullshit?"