I agree with this, but I'm still willing to give them the benefit of the doubt for the time being so we don't spend kills on them until we have to. At least if they are the mafia team, we are (in theory) safe waiting a bit since we only need one to find out who most or all of the team is. If they are mafia, I imagine it went down something like this: Natty decided to make the claim, but it was flawed. So then the other two scummates claimed to save her from looking suspicious. We'd have to believe that Natty was capable of making a mistake like that and dragging two scummates down with her, but I find that hard to believe.
That seems entirely plausible given the amount of time it took for Natty to reveal everything on account of "they didn't want to be modkilled". And I suppose stealing a diamond after death makes some sense, thieves are a bit short on the honor department.