Okay, Murata did something really bad at the end of Kyou Kara Maou episode 75 and this is my theory as to why he did it:
Murata is definitely up to something. Remember all the times that he acted all innocent-like after sending Yuuri back and forth from Earth to Shin Makoku? He is extremely well at deception.
My thoughts on what Murata is up to have to do with how Yuuri dealt with Soushi at King Belal's castle--instead of fighting force with force, like Yuuri had done the night prior against those possessed spirits that attacked Adlebert's fort (and remember that the two were basically equal in strength, at a stand-off before Soushi withdrew), Yuuri used his love to defeat Soushi.
Now Murata knows that fighting Soushi's with force doesn't work--Shinou is proof. Yes, Shinou managed to defeat Soushi and lock his power away, but not all of it, and that last little bit is what infected him. And, Shinou being who he is, fought the possession with all his strength, trying to use his force of will to make the thing leave him or at least not to gain control. I think Murata has finally realized why Shinou, the founder of Shin Makoku, the great original king, could not win against his own possession and Yuuri was the one that showed him the truth.
Before I get to that, am I the only one that thinks Soushi is now trying to possess Yuuri? I mean, Shinou's soul is cool and everything and extremely powerful, but he doesn't have a body to go with the soul, and if Soushi is going to do his world domination thing, he is going to need a body to move around in and hold a sword with. Yuuri, being the Maou, is the strongest person in Shin Makoku, with perhaps the exception of Shouri--I imagine that once he gets a hang of his powers that he will be just as powerful as Yuuri, but he can't handle his powers--his body can't handle all that power at once, so he is going to have to learn to use it slowly, like Yuuri did--so that rules him out as a host for Soushi.
That leaves Yuuri, and Shinou/Soushi is planning on doing the same thing he did to Shinou when he was alive--infect the body and corrupt both the body and soul, and once they are both corrupted enough, Soushi will take control of the body, like he was in the process of doing with Shinou before they sealed his soul away. Murata has figured out that this is Shinou/Soushi's plan, because a body-less soul is of little use to an entity hell-bent on conquering the world and since evil guys are never very creative, if something worked once, why not do it again?
So, Murata first got Shinou to give back the keys--something that Shinou did not have to do--and then helps trap Yuuri while Shinou ejects the rest of them. This guarantees: that it is Yuuri that Shinou/Soushi will try to possess and not someone else, like Gwendal or Gunter; that Shinou will have no one else to hurt while trying to possess Yuuri; and leaves Yuuri with no distractions. Well, until they go back to the castle and get the distraction once he wakes up. Ejecting the others also has the bonus of keeping them from further harm and getting them return to the castle without further argument--if they can't go into the original king's shrine, then they have to go back to the Blood Pledge Castle to regroup and pick up Wolfram.
So that explains why Murata helped get rid of the others--well, also because what Yuuri has to do now they can't help with and would only be in the way, getting themselves hurt trying to rescue Yuuri and distracting Yuuri while Soushi is trying to possess him.
Now, the big question: Why is Murata aiding Shinou/Soushi in trying to possess Yuuri?
Simple: Murata has total faith in Yuuri--he knows that there is no way for Soushi to defeat Yuuri. Now, even with this reasoning, it doesn't explain why Murata didn't sit back and let the conflict go about naturally--until I realized exactly how Yuuri has to defeat Soushi--it has to be exactly like he did at Belal's: not by fighting back but simply with his love.
Violence begets violence, no matter how well intentioned, no matter the reason behind the violence, it is simple: violence is violence no matter how you package it.
Shinou fought his possession with all his night, trying to defeat Soushi as he had on the battle field, but by trying to fight the possession, Shinou was actually making Soushi stronger--he feeds on violence and anger. So Shinou, by fighting the possession as he did, only made the possession that much stronger. Fighting Soushi will never work, simply because fighting is a form of violence, and violence only begets violence. The only way to free Shinou of Soushi is for Soushi to leave the soul willingly, which would never happen, unless there is a better host nearby.
Murata knows that because all 4 boxes weren't unlocked, Soushi isn't at full power. I'm sure that Murata will also reassure Soushi that he will have no problem in possessing Yuuri, that the boy is weak--never fighting back, always trying to talk to the people attacking him. Now, Murata really knows that these aren't weaknesses, these are actually Yuuri's strengths, but Soushi and Shinou will not see that--both come from a warring time (well, really everyone in Shin Makoku is, except Murata and Yuuri) where showing any type of weakness, such as a unwillingness to fight back, gets you killed. Strength, to both of them, is measured on the battle field--either you are strong and survive or weak and die. Shinou/Soushi know that Yuuri has great power within his body--hell, Shinou is the one that made him Maou and gave him that power--making him an excellent host. They also see his unwillingness to fight as a sign of a weak will, something that they will conquer easily.
Back to the boxes: only three were opened, so Shoushi is not at full power. That means that if he intends to fully possess Yuuri, he will have to use all his strength to do so, meaning that he will have to entirely leave Shinou--not leaving even a small part of himself in the original king because he will need everything to totally overtake Yuuri. Murata has finally found a way to free his friend's soul of its taint. Of course, this sounds like he, again, is betraying Yuuri because he is trying to free his friend's soul by letting Soushi take possession of Yuuri.
And Murata is, in a way, doing exactly that--he is trying to get all of Soushi out of Shinou and into Yuuri--not because he wants to free his friend's soul (it is a nice side effect though) but because only Yuuri truly has a chance of destroying Soushi. Murata knows that Shinou can't--he has had 4000 years to do so and he couldn't do it, probably because a little part of Shinou's brain told him that having all this power is good, it makes him strong and he has to be strong. After all, Shinou was never above a little destruction if it was for a good cause.
Yuuri, however, never wants destruction, never wants violence, never wants to force someone else to do what he wants, so there is nothing in Yuuri that will want to keep Soushi's power, nothing about Soushi will appeal to young Maou, which means that there is nothing to keep Yuuri from totally rejecting Soushi. And I bet, given the chance, Yuuri will try to get along with Soushi instead, ask him why he wants to take over the world and then be his friend. Yuuri will not want to destroy Soushi, he just doesn't have it in him to want to kill anybody, even if they are an evil tyrant.
Once Soushi totally enters Yuuri body, gathering all his strength together--which means leaving Shinou completely--only then will he make a bid to take over Yuuri's body. And only then, when he tries to fully take control of Yuuri, will Soushi realize that he can't (or so I suspect). Violence, death, destruction & chaos are what makes up Soushi and none of those things are a part of Yuuri. When Soushi goes to take control of Yuuri's mind, to manipulate Yuuri into letting Soushi control his body, he'll find nothing for him to appeal to, nothing for him to use to plant a seed of hate and violence. He'll find love and peace and naivety--all concepts totally foreign to Soushi, all things that he has no use for, things he sees as weak, things he can never understand. And if you don't understand something, it makes it really tough to manipulate it.
And when Yuuri realizes that Soushi doesn't understand love, or peace, he'll try to explain it, share it with him, try to make Soushi into a better person. Yuuri will feel nothing but love for Soushi and this love is exactly what defeated Soushi at Belal's castle. This love will be something that Soushi will not be able to stand and if merely being in the presence of Yuuri's love wiped out his puppets at Belal's, what do you think being inside that love, surrounded by it, will do to the evil spirit? Either Yuuri will totally dissolve the spirit, leaving nothing left, or dissolve all the evil, leaving the cleansed spirit of the first man, the one that fought Shinou so long ago, and then set his spirit free, along with Shinou.
Wow, that is a really long theory--but it is sound and I'm sticking to it.