I could think of many ways to make the downfall of rapture and not do horror. It'd pretty much be the start of rapture, showing Ryan as this great leader building a place for science. Then you'd have the plasmids introduced with people using them for the common good, but after a short time there'd be one or two "mysterious deaths" and we could follow Sullivan around (he did security and some dirty work for Ryan, but was morally against it all) Eventually, there'd be riots and we'd "see" Fontaine shot dead. Throughout all of this, we hear more reports of deaths and occasionally see people going insane from the plasmids.
The last bit of the movie would be Sullivan following Anna Culpepper around and being forced by Ryan to kill her. So at this point we pretty much hate Ryan and know he's the bad guy. Throughout this, we start seeing grafitti saying "Atlas Lives" Then after Sullivan turns in his badge (and/or kills himself) out of remorse for turning into a bad guy, we see a mysterious plane crashing... end of movie, wait for sequel
