Well... I think it being Denji was part of some plan? Because they'd protected him.
[So it's technically GOOD it was Denji because otherwise someone would have actually been executed.]

But... Yeah, I don't really like how it feels like there's this big group of murderers who kept secrets from everyone else so they could save the day. Even if it worked out. It just feels like nothing's changed since I was at home. People keeping secrets from me to "protect" me, and then I end up being useless.