Hmm, you know in the first game I almost started to feel bad about killing splicers after I found out that they were attacking me because they were slaves to their ADAM addiction induced insanity; an addiction which many of them were forced into because of fear following the start of the civil war. They were tragic figures, and I really hoped that at the end of the game there would either be some sort of cure or else the city would flood and end their suffering (I still think the reason that Rapture wasn't destroyed at the end of the first game was because marketing pressured them to leave room for a sequel).
Unless I'm mistaken it looks like the citizens of Columbia are slaves even more so. You see a man in a cart with a broken axle, a woman sweeping out her store while it burns and a politician stumping before an empty park. My guess is that something is projecting an idealized vision of Columbia on these people and not allowing them to see the corruption and destruction that has occurred. And the only reason that these people are attacking you is because their perception of your character has been altered to make them hostile to you. Or maybe it's the other way around? Maybe you are attacking them because your perception has been altered? The fact that you are seeing objects shift makes me think that Booker's perception is being at least somewhat altered.
Huh, maybe that's going to be the twist this time around? Maybe Elizabeth (or some other entity) is the one responsible for what's happening and she's treating the citizens of Columbia like her own personal pets. Pets which, after years of psychic control, are finally becoming immune to her psychic impulses and are able to break free momentarily in order to attack her and her pawn: you, a new-comer to the city who has yet to develop an immunity. It would help to resolve the whole "killing slaves" concern in my head and would actually help develop motivation for killing Elizabeth if she (or whoever turned out to be the "man behind the curtain") turned out to be the super-secret final boss.
Far more motivation than I'd had for killing Fontaine in the first game anyway. It never made sense why he'd want to kill Jack, his own personal slave/pseudo-son. The only motivation I had for killing him was because he was trying to kill me and a sense of betrayal which I overcame fairly quickly.
On a lighter note, the turn of the century was the height of the American Big Band movement, when people supposedly actually liked going out and watching parades with marching bands playing Sousa tunes. So how awesome would it be if you came across one of those parades playing something like "Stars and Stripes Forever" and then they suddenly became hostile and started clubbing you trombones and such? SUPER AWESOME? Yeah, I thought so. Plus it'd help reinforce a lot of the themes they seem to want to convey: patriotism, jingoism, turn of the century Americana, and general f'ed upedness. Make it happen Irrational!