Thinking about how the prospect of how Jinu sacrificing himself wasn't even on Gwi-Ma's radar.
The night before, Gwi-Ma had completely broken him - The freedom he thought he could finally attain after so long? Gone. The monster that Jinu saw himself for 400 long years? That's all he could see more so now than ever before.
But more importantly, he lost the one person that Jinu finally found solace in - the one person who truly understood him - Rumi.
Everything gone because Gwi-Ma had tormented him beyond repair - threatening a further eternal life with those intrusive thoughts louder than he could possibly handle.
He was already at wits end as it was.
Now Gwi-Ma had him. Clutched his fiery claws into him. Jinu was once again the good little solider that Gwi-Ma needed.
Not even Derpy, Jinu's most loyal companion and best friend, could get through to him. Jinu had always cuddled Derpy, but now, Jinu simply stared ahead, lost in depressive thoughts. Completely broken.
For context, this is Jinu and Derpy before Jinu was tormented:
Jinu's little smile after he finishes petting Derpy lives rent free in my head:
Anyway, so Jinu is doing everything he is supposed to do for the Demon King after Gwi-Ma had tormented him (The Idol Awards, Your Idol etc) and victory looks all but guaranteed.
One small detail you might already know as quite a few people have pointed this out already but after the Your Idol sequence, Jinu is the only Saja Boy with his eyes open (well with Mystery we assume his are closed behind his hair alongside the other Saja Boys but its very likely considering the symbolism).
Jinu looks absolutely terrified. He is scared. He doesn't want to do this, but given the circumstances, he feels he has no choice.
So anyway, a wild Rumi appears from the shadows after jamming to Your Idol (lets be real, she could have entered the arena at any point but she chose the end of the song because its a banger) and suddenly, a showdown between Huntrix, Gwi-Ma and his demons occurs. (I phrase it like this because the demons are slaves to Gwi-Ma, its basically a hivemind.)
One thing to note is patterns can be used for Gwi-Ma to control his demons like puppets - we literally see this at the start of the film.
The flight attendant demon is forcibly tugged up to Gwi-Ma against her will using her patterns. Beforehand she is apprehensive to go to the demon king - she is scared. But he forcibly pulls her up before yelling at her for her so called failings before incinerating her.
This is not the only time we see this.
Whenever Jinu is pulled down to the underworld, his patterns rise quickly before he's tugged down.
Gwi-Ma can control demons through their patterns.
This is very important information going forward.
So the battle rages on and Rumi is dead set on saving Jinu - Jinu who has not moved to fight her even though that is what he was meant to do.
But Gwi-Ma is also aware that Jinu is powerless - he's still broken - standing before Gwi-Ma as if protecting him. (You can vaguely see his shadow in this screenshot.)
Jinu is completely helpless.
Not only had Gwi-Ma threatened to make his life a living hell, worse than it already was for the rest of eternity if he didn't comply, but he can literally use Jinu like a puppet through his patterns - Jinu couldn't help Rumi even if he wanted to.
Plus, Gwi-Ma is at his most powerful. Even as it stood, there was no way the girls could defeat Gwi-Ma in this state.
Jinu is mostly powerless - broken - but he also couldn't stand and watch as the woman he loved got killed by the devil he despised.
So while he was powerless - while he couldn't fight alongside her like he wanted to - he helped her the one way he could:
"Dive in the fire and I'll be right here by your side."
He might have been broken - powerless - but he was always going to defend Rumi.
Gwi-Ma was defeated at his own game.
He'd completely underestimated how much Jinu loved Rumi.
