I think it’s Della Guardia. I think Molto is guilty of trying to fake evidence to help secure a conviction but doesn’t know about bosses involvement. Victims was scared of someone at work. Victim knew some secret that would cause Della Guardia to lose election (very plausible since she seems to be willing to do anything to advance) He’d be in a position to know enough about the old murder case to try and set the scene to look like it was a past defendant with a grudge. The fact that Rusty fell into his lap as a suspect could simply be serendipity. And in theory it would explain why the case would be rushed through before getting all the DNA sample results etc. And he could also have some dirt on bartender if bartender did spike Rusty’s drink and it was an intentional set up of Rusty. When you think of all the major players involved, he’s the only one the writers haven’t pointed the finger at in any way leaving it the “biggest surprise” at this point.