I gotta make a distinction with something. Comstock isn't Booker. Comstock is what Booker might have been(although yeah the whole Booker/Comstock thing and how that ties into his arc is the most interesting thing about the game). The weird thing about the game is that Columbia is a symbolic exaggeration of Booker's terrible past, but he's also the most stand up guy there. It never feels like he's coming to terms with his past sins so much as just fighting bad guys, even after the reveal.
I don't see Columbia as symbolic of Booker's past. I see it more as emblematic of the way he rationalized his wrongdoing in another timeline where he became Comstock. The city of Columbia is the result of "rationalization," while Booker's sequestered existence in his apartment is the result of "depression" (used here to mean living with guilt).
And as far as race, I never really got the feeling that Booker was all that racist to begin with. Yeah he took part in atrocities but that's not quite the same thing as just out and out hating black people(in general, I feel Booker becoming Comstock is some serious Stretch Armstrong shit). Maybe there are some audio logs I missed and all, but he seemed to act pretty courteously to all the minorities you run into. Because of that, it doesn't really seem like an essential part of his arc and just something there to provoke. The people of color in the game are merely props to facilitate Booker and I'd kind of rather they just not be there at all in that case.
Booker -was- racist, back when he was a 20-something at Wounded Knee; he killed partly out of racism, and enjoyed it, and partly to prove himself to his fellow soldiers (since Booker himself has Indian blood). But here's the thing: The Booker we play we as is no longer racist. This is the "catch" to Depression Booker, the Booker who rejected baptism and believed himself irredeemable; while he is trapped in a vicious cycle of drinking and debt, he at least he did not rationalize away his wrongdoing. Instead, he lived with the guilt, and since he lived with the guilt, he eventually came to see racism for its inherent ugliness. He never stopped feeling guilty, and that guilt inhibited further racism.
Now Rationalization Booker, aka Comstock, did the opposite: Baptism "washed away" his sins, and born again as Comstock, he not only felt forgiven what he had done, but justified in it. He came away from baptism with a new sense of self, a newfound charisma and confidence and
conviction that attracted followers and reinforced his racist beliefs as more and more people echoed his cult of personality. He became politically influential, and, unbound by any pity for minorities, he steamrolled them. He became the opposite of Depression Booker -- he became
somebody, somebody who has
everything, everything but a sense of remorse for past ills and any desire to do better.
I've seen people do this in real-life. Not to Comstock's extreme, of course, but I've seen people "born again" who become more bigoted for it. It's terrible, and a result of ideology whitewashing one's past and filling in thought and reflection with dogmatic absolutes.
I really can't agree about Columbia seeming idyllic. It's pretty yeah, but as soon as you get there you're forced to take part in their religious rituals(nothing against religious folks), you overhear people talking about Comstock's divinity, watch people literally worshiping Benjamin Franklin like a god(red alert bro, c'mon) and see how they make monsters out of men. That Handyman clearly doesn't wanna be there, I mean he's pulling a Frankenstein routine right there. They concoct inhuman abominations in labs you don't wanna raise your kids here.
OK, that's true, there is a bit of eyebrow-raising stuff, but at the same time there is abundant sunshine, greenery, glorious monuments/architecture, shoe-shiners, bakeries, creameries, candy stores, cafes, restaurants, gift shops, cobblestone streets, kids playing in a fire hydrant, people picnicking, parades, flower boutiques, and so on, and so forth. It's just a quaint and peaceful place.
The forlorn Handyman aside, and ignoring any personal misgivings about cults for the moment, there is just an idyllic state of existence in Columbia, one not unlike depictions of the "good ol' days" of the Deep South. Of course, like the Deep South back then, the shiny exterior conceals an ugly underbelly of racism... I felt it was well-done.