You were just another infected, but there is something that differentiates u from the other infected, you DID have a conscience

You were wandering through the destroyed streets of the city until you saw two bloaters attacking a girl with reddish brown hair, you barely saw her and you fell madly in love with her, so you ran to help her, knocking down one of the infected

After the two of you killed the infected, Ellie quickly pointed her gun at you and was about to shoot until you spoke