Mostly really liked this episode, but not entirely. There's some really clever twists, as well as a HUGE change to a major character, but there's a decision early in this episode that threw me in a way that soured me some on this episode.

I don't know why they didn't bring the Ryuki vs Knight battle to some kind of a conclusion.

It's a choice, and, short of killing off one of the two main characters with fifteen episodes left to go, the only choice they could make. Fight's got to end without someone dying. It's just, I'm shocked that there was zero effort made into showing why the fight stopped. Did their time run out? Did Ryuki have Knight on the ropes and split? Were they both too battered to continue? To have a fight this big, this important, be left to the viewer's imagination... man, I don't get it. I don't know why you wouldn't spend a few more shots tightening that resolution up some. Weird choice. Unfulfilling choice.

The rest of the episode was pretty strong, though. In contrast to the big Hero Fight, the rest of the battles had some thought put into them, some smart ways to leverage characterization to conclude a fight. The Zolda vs Ouja one, so great. Of course Kitaoka had a plan the whole time. Of course he wasn't going to fight Asakura unless he had some advantage. Using the police to corner Asakura, in a specially chosen reflection-less alleyway, it's a genius move. It's nice to see that Kitaoka cunning brought to bear in non-murderous ways (although I'm sure he's betting on Asakura being devoured by one-to-three monsters), to make his successes feel more laudable. I like not feeling a little conflicted about cheering him on. It's good to have him winning in ways that aren't completely scummy.

Also, he didn't murder anyone, which makes him more heroic than Ren.

So, yeah, this is the one where Ren Murders Someone. (Right upfront: I don't think Kamen Rider Boss/Shiro is really dead. Don't buy it, not for a second. That said, the issue isn't whether someone died, it's whether Ren murdered someone, which he absolutely did.) It's been a long road to get here, with a lot of twists and turns. There've been times where it seemed like Ren would kill every other Rider without blinking an eye, and there've been times where it seemed like he'd given up on trying to be a killer. Now, after fighting Shinji to a standstill (allegedly!), and finding every other Rider taking a personal day, he's got no one left to fight but Shiro. It's suicide, but he's going to do it anyway.

Ren's a lot like Shinji at this point in the story. Shinji's plan in this one is to maybe find Shiro, maybe fight Shiro, destroy the Mirror World, something. He doesn't have a good plan, so he goes with his bad plan. Ren's got that same mentality here. As Yui points out, there's no pathway to victory for Ren. Even if he becomes a killer, there're too many Riders left to kill and not enough time to save Eri. It's pointless. But Ren's like Shinji, he can't ever do nothing. Ren can't concede defeat, can't abandon Yui to failure. He'll attack his best friend, try to become a killer, even throw his life away fighting a nearly-all-backhand godlike Rider, just to feel like he's doing something. He needs to try, always, at any cost. But, unlike Shinji, who more-or-less lucks into a way to save Ren by learning of the existence of the Second Secret Science Society aka Kamen Rider Tiger, Ren uses strategy and tactics to defeat Kamen Rider Boss. No, not defeat, murder. He murders him. It's a point of no return for Ren. It's the biggest decision this show has made, to spend 30-odd episodes telling us that Ren killing someone is The Line, and then crossing that line. It's shot perfectly, with Ren's triumph being framed as the eradication of his soul, of a psychic wound that's likely fatal.

And, cherry on top, it was probably all for nothing, as a tear-streaked Eri opens her eyes in the hospital. Ren became a killer to save Eri, who didn't need saving after all. All his efforts did was make him a man she's ashamed of. So pyrrhic. So perfect.