I'm new to the board but I've been an avid Assassins fan for about five years and studying the show. I know Joe Mantello and have had a few conversations with him about the show as well.
The Balladeer and the Proprietor:
It's not as clear cut as the 'good' and 'evil' ones. The Balladeer's purpose, especially in the beginning, is to be someone the audience can identify. Generally the audience won't invest themselves, onstage, with the people they see as wrong or 'evil' so the Balladeer is someone much more like the average person. Though, also like the average person, he often mocks the Assassins for their 'excuses' putting them at a lower place and yet eventually becoming their ultimate spokesperson, so to speak.
The Proprietor is similar to a ringleader for the Assassins. He's very much more an abstract than a real character. Their little spirit to give them a hand here or there - the accomplice that can't be connected to the crime. Someone here said that the extention of the character was an experiment which it very much was. The fault in the experiment is that, as written, Booth does a similar, egging on to the rest of the Assassins. But nothing can be perfect... In the reprise of 'Everybody' the Proprietor stands to the side and watches the assassins, similar IMO to an artist admiring a finished product. One thing that I also thought was brilliant is that Marc Kudisch (Proprietor) does all the 'voice-overs' in the script, including an amazing Reagan impression after Unworthy. This makes the lines much more potent than just having the voice.
I know this is getting long BUT
A few obvious themes of the show are understanding/perspective, Love, Who is Ultimately True and the more underlying theme Grief.
And, of course, what really is the american dream:
Before Marc actually begins singing 'Everybody' the ensemble (in all white - they are always in all white costumes) stand at the back of the stage waving, etc as confetti falls creating the Postcard Perfect American Dream - quickly followed by Marc's aggressive confrontations with the assassins about their own dreams.
Many of the songs are just trying to show the humanity in a type of person that is shrugged off as being "An Assassin" and not a person.
One thing that I find interesting (or should I say one more, lol, I'm sorry but I do love Assassins and I've been analyzing it for a while) is that one of Sondheim's most melodic and beautiful songs, Unworthy of Your Love was written for two attempted assassins to their unhealthy obsessions. -Just food for thought.
I'll stop now. But I hope I helped a bit!
Do not do unto others as you would that they should do unto you. Their tastes may not be the same. ~G.B. Shaw