@Kayriel
I see that you've been defending the girl's plan, so I'm going to offer a few counter points.
1: Why would a warrior race ever want to attempt diplomacy for the return of one of their strongest fighters? He lost; he's dead to them. Even if they did want him back, any faction like them would be pissed with the Demon Lord's methods.
"You can't beat us, so you take a hostage?" Just imagine the perceived insult.
2: The Demon Lord just isn't suited for the position. She's literally just a kid who doesn't understand the concept that not everyone wants to co-exist peacefully.
There's no recourse from that, if a faction doesn't want peace, you either have to back off because you aren't prepared to fight for it, or grow up and realise diplomacy only works when every party wants it to.
She
might be a good ruler during peace times, but she's categorically incapable of bringing about peace during war times. It's not that she wants what's best for everyone - that'd involve understanding that maybe some people aren't able to reconcile - she just wants to force her ideals on everyone...in the most passive way possible. Look at how many people have died and she's made zero progress.
What is she going to do if someone says no? Cry? Or wait for her subordinates to get shit done? It's Naruto all over again but with none of the self-awareness, at least he got that some people
have to die.
3: The "good plan" you're talking about never even existed, she came in and declared Hiro not kill him simply because she didn't want him to. Hiro asked why, and her right-hand had to swoop in with a justification, which was 100% on the fly otherwise he'd have already tried to restrain the beastman.
She wanted him to live because she isn't fit to rule. She wants peace, but isn't prepared to fight for it, or even let others do so in her place; everyone has to get along because she said so.
That is why this was idiotic. It's not because of what happened, it's because of why, and the fact that it could've easily been avoided.
Like someone else said, any lives lost here are on her. I know if it comes up then her aids and Hiro will coddle her and say it's not her fault, but it definitely is.
I don't even dislike idealism; it's the fact that most idealists aren't actually prepared to work towards their vision, they just expect everyone else to change because they don't like the way things are.