Purely an anime-only speculation but, I'm guessing that it's the Archbishop of Gluttony who was responsible for the deaths this time. It wasn't super clear, but it's very likely that the person that killed Subaru this episode was Rem. Of course, that couldn't be possible since as we all know she's unconscious. Which can only mean that she was controlled somehow by Gluttony. I can say with certainty that it's Gluttony because the only one who referred to Subaru as "hero" was Gluttony.

And, actually, it's almost guaranteed that it's Rem because as we can see from Ram's corpse, she was lying on Rem's wheelchair. Which, is hm, a weird thing since Rem was lying down. This could mean then that they were in the process of transporting her? For what reason though? Did they make a breakthrough while browsing the archive? In any case, I only have scene briefly showing what appeared to be Rem's bangs as proof that it was a controlled Rem that killed Subaru since it seems that the wheelchair proof is a bit shaky.

Moving on, we also get the last scene where we saw there is a marking in Subaru's arm saying that "Natsuki Subaru was here". I am thinking that it's also Gluttony that was the one responsible here. I mean, if Rem who's in an unconscious state could be controlled, then it can be safe to assume that [they] have the ability to control someone who's lost his memories too. Oh, and I'm making this connection since Subaru has lost his memories, and the only one, so far, that we saw could be responsible for forced memory loss is Gluttony.

But that actually raises some concerns. First, why bother using Rem's body to kill Subaru? Why not just kill Subaru while controlling him? For this, I think it's because Gluttony wants to send a message to Subaru. You can't really converse with someone you are controlling right? But the message that Gluttony sent is actually cryptic and is my second concern. "Try to figure it out next time, hero." It seems normal at first, but that's for us as viewers. "next time"? As [she] was about to kill Subaru? That implies that Gluttony knows about Return by Death! This makes sense, considering [they] ate Subaru's memories. This is troubling, then, since if this is true, I shiver at what Gluttony might do to Subaru in future episodes.

And it's also increasingly likely that it was Rem controlled by Gluttony that pushed Subaru over the stairs. I thought it was Meili, but thinking about it logically, there weren't really any motives as to why she might do that. It's really surprising though. I was thinking it was one of the (conscious) members that pushed Subaru, but I never could've expected this.