If what the general said about the barrier is true then the kingdom was always planning to kill Takeru which is just sad. You summon someone against their will to fight your battle for you and then you plan to just off them after their job is done.
Also, the female hero was right in a sense that his power would bring ruin to the country considering his death allowed the demons to invade and the country ended up in ruin...so serves them right now.
From what we got based on the flashback during the meeting with the Demon Princess
(Chapter 37), seems that the plan to kill the Messiah was laid by something external altogether (not Demons).
Not the King or the Kingdom, but pawns of this External Influence like the Prime Minister and possibly even the Three Heroes.
I mean, as mentioned in this Chapter, the Prime Minister was apparently even holding the King as a hostage against the General and planned to kill even him from the start.
They even put up the Kingdom wide barrier in order to not only stop the Messiah from acting up, but also to stop any Civilian who tried to help him (who weren't misled with the false propaganda as we saw during Messiah's execution).