Real bro
It's likely because it wasn't possible to save his life at that point. The non-stop fighting under the control spell along with the years of torture he likely received after he was captured were too much for his body to endure anymore. I suspect the MC cured his mind so that he could be himself for his final moments. He wouldn't have been able to say goodbye to his younger brother and the younger brother would have never been able to find closure if he died while still under the control spell. As for why he asked his brother to kill him... They said his internal organs were basically shredded, so he probably was only going to be able to live in total agony for a few moments longer. Rather than bear the pain just to live for a few moments longer, he probably wanted to see that his brother had gotten strong enough to not need to rely on him anymore like he did when they were younger. He probably wanted to have some kind of proof that his brother would be okay after he was gone in addition to wanting someone to end his suffering.
That’s what I was thinking too but the hopeful side of me hopped he could’ve survived somehow. （/TДT)/
Dude is like “Oh Hwi you have grown up, good.. now kill me with your sword” why bother cured him.
Can you just live and watch him grow. Hwi went through so much to save you he even increased his payment to mc