To further add to how Kusaba could still be alive:
In real world shootings, only about 39% of people hit by .38 Special, which I assume is what the officer was shooting, are actually incapacitated with a single shot.
It's ultimately not a super powerful cartridge, meaning that if the bullet doesn't directly pass through a vital organ, in many cases it won't actually be lethal.
The revolver in this case also had a relatively short barrel, so it won't have had a very high muzzle velocity, even compared to other guns of the same caliber.
Keeping that in mind, and looking at the exact shot placement in the last chapter, it isn't implausible to say that the bullet could have ended up lodged inside his rib cage without ever reaching his heart or lungs.