That's just... How storytelling works? Of course they straight up used a character in this story to serve the development of other characters within that story, that's what they're there for. The character of Liza was created to inform the rest of the cast about how the infection works and to further the development of Travis and Chris. She was a fine character, but she played her role in the story, and now is no longer part of it. That's not a mistake, that's character development.

Or next you'll tell me Patroclus' death was a mistake because it was used to further Achilles' storyline.