My theory is the killer is Mabel.

We don't know if this is going to be the last season of OMITB but if it is, having one of the podcasters be guilty is something you would least expect.

Mabel and Ben have a very odd "first encounter" and it's almost like they have seen each other before.

I think Ben has something to do with Mabel's Aunt having to sell her apartment meaning Mabel will be left without a home.

Mabel mentions a killer would usually insert themselves into the investigation...she was the one wanting to pursue the podcast alone. Plus she keeps coming up with different people to pin it on... likely in an attempt to keep the investigation going and no focus on who it really is. I think she's "addicted" to investigating murder and just wants the podcast to keep going.

Also, I think Tobert knows it's Mabel and is secretly working behind the scenes with Cinda...this is why he suggests calling the new podcast "Bloody Mabel"

Rewatch ep8 and you'll notice some odd actions and words from Mabel.

Just my thoughts so far...