I actually agree on the conspiracy theory front. That knife is such a Chekov's gun, and I already thought that person might be linked to the stuff that mysteriously appeared in the morning. My back-up theory, which is probably more likely, is that a certain possessive someone targeted them to become a demonheart so they could protect someone else. Though now I've just typed that, I realise it describes two different groups, so I'll say it's not the group suspected in the chapter. I find that person surprisingly untrustworthy, although maybe that's because I kept pursuing Kaiser.
And thoughts on a certain someone at the end? The last conversation between the Master and Raze implies they're not a demonheart, which I seriously thought they were even after someone self-confessed. Specifically, that the self-confessor was the actual killer getting away with it while they were one of the starting demonhearts. Hell, maybe that still is the case and they're just being creepy for the hell of it, certainly their other partner this chapter had their moments too. I had a fair bit of approval from that person right at the end, I think, though it'd be nice if we could visualise it somewhere (I thought that was an option but couldn't find it?). I'll be curious to see how differently they behave on a full disapproval run, that's for sure.