I don't remember the specifics to be honest. All I remember was a scene where he and the female lead were about to be rescued by a helicopter on a rooftop, but he decided to stay behind because he thought he'd been infected. There was something about a gun that either he kept to commit suicide with or she realized he gave her the gun, essentially saying he wouldn't survive and need it... She breaks down as the helicopter is leaving and feels broken, completely expecting him to leave with her after they'd survived through a lot to get to that point (though I don't remember what all lead up to it).
It had a similar feel to "Zombie no Afureta Sekai Ore Dake ga Osowarenai" if I remember correctly.

I'd greatly appreciate any ideas as to which series it may be!