This kicks in at the ending, where the cure works, except some people were fatally injured by the infected to make more zombies and those ones will probably drop dead. And then there's the psychological trauma that some of the cured infected will have when they realize they have attacked and probably eaten multiple people.
There's also the fact that the heroes operated on the logic that the whole city would probably have to be wiped like in previous outbreaks, and thus were liberal with explosives and high-grade destructive weaponry to deal with threats as they came. And personally killed a large amount of A-Virus victims themselves, mostly in self-defense. The fact that the virus could be cured meant that even with New York City saved, the BSAA were responsible for a lot of innocent deaths, especially with the railgun Nadia used that moderately wounded the Big Bad - and wiped out several buildings worth of innocents.
It says a lot about Leon's Took a Level in Cynic that he tosses around live grenades and narrowly avoids a fatal crash from a car pileup on an active highway without even a single second of hesitation or looking back, considering his nature of always trying to save everyone he can from every story prior.
Leon was in both Tall Oaks and Lanshiang, he knows exactly how bad a viral gas attack can be, his failure to save both the president and both populations probably is what caused his breaking point.
This film was supposed to coincide with Resident Evil 7: Biohazard, but ends up irrelevant to that game's plot overall. However, it ends up being accidental Foreshadowing for its sequel, Resident Evil Village. The sheer resources that the B.S.A.A. put into capturing Arias before the A-Virus even took over the entire city, causing mass destruction and even punching a hole through an entire skyline, makes 'sense' before they find that there was a way to cure the people. But their He Who Fights Monsters morality ends up making their willingness to capture and use B.O.W.s combined with taking any measures to complete their missions while covering everything up seem like an extension of their reckless and destructive nature in this film.
