And there we are caught up with the anime.
And the explanation is exactly the same. Humans and demons have decided that they are now powerful enough that hero and demon lord are irrelevant.
So, humans and demons agreed on killing the hero because she's "obsolete", before even finding the demon lord and gauging his strength. And his intentions, which were, funny enough, to protect the hero. They all decided that a hero and a demon lord are not strong enough to replace an army. Or even make a significant contribution. And that war can now take place without these two "rounding errors".
Great. Tell that to Seika again. To his face. After he just trampled a whole army. Without laughing at your own ridiculous explanation.
In his previous life, he was killed through the betrayal of one he trusted. He was otherwise too strong to be killed directly.
Here, he is even more powerful and they were about to kill one of the few he would trust enough that they could kill him.
Great plan.
Worst part?
Seika is probably one of the most dangerous type of demon lord. Human appearance, so he can sneak into most human locations. Also human thinking patterns overall, so he wouldn't even stand out from alien behaviors. Can summon devastating monsters for a cataclysmic strike, then disappear before anyone can intervene, or even identify him. No need to parade around with a whole army that will announce his presence hundreds of miles in advance. Can use a form of magic that nobody in this world knows about, much less know how to counter. And much harder to kill than his short human body looks like because of the substitution paper dolls. He also has divination abilities: though not on the same level of precision as the princess, he could likely use them to throw uncertainties in her plans, should he intend to antagonize her. And curse abilities that can trigger at extreme long range.
Not completely invincible, but, by the time you manage to setup countermeasures against all of these, the death toll would be more devastating than a straight-up war would be.
It's almost like the humans and demons would count on a demon king that bluntly marches into a warzone to meet with an army head-on. Which they still miscalculated, but was a dumb assumption to make in the first place.
So, the humans and demons prepared a whole conspiracy to discard their best possible trump card in favor of a plan that ignored the most important factor of all: the demon king himself. And I included the demons above because apparently even they didn't want to be led by a demon king. Which makes it all the more hilarious in retrospect when the assassins actually encounter him and realize their mistake. "Great, we came to a point in history where the demon king is a negligible variable so we came to kill the human hero... who happens to be his best friend and he's totally not a negligible variable."
(Sidenote: to those who suggested in the comments of previous chapters that the demon assassins could and probably would have sided with Seika had they realized earlier that he's the demon king... now you see how that wasn't an option at all. If anything, he was as much their target as the heroine. They just miscalculated how much of a threat the demon king actually is. By several orders of magnitude.)
And here is one cunning princess who thinks she could make things work out peacefully... to some extent... when she could have derailed the conspiracy with a few simple actions. Like "coincidentally" inviting the heroine for a tea party the day of the assassination, providing an undeniable alibi. Or telling Seika about the assassination attempt and its fallout so he could simply disable the demon "diplomats" and send them back when the accusations of killing them came up. I can think of tons of ways to derail the whole plot, possibly even exposing it publicly so it can't bite them back later, without triggering Seika's rampage. And funny enough, the one thing she didn't go so far as to predict that would have made this timeline closer to a logical choice is that he would erase the traces of his rampage, down to the lives he took.
She has a cheat ability that she obviously abused to check multiple possible futures, but all she could do was the same BS as Avengers: Infinity Wars + Endgame? "I checked millions of possible future and this is the only one where we win." Dr Strange and her are the same: so unimaginative that, despite a cheat ability to see the future with control and clarity, they can't make a plan that makes sense and only manage a win based on a timeline requiring so much luck and so many contrivances it shouldn't even happen.
Basically, the only element she used her ability for was finding a way to be something close to a friend to him so that he wouldn't kill her on sight. Which worked, but she apparently disregarded the cost of everything to that point.