She's playing the role they created for her. It could be that the role was miscast as sometimes happens with TV shows. Andrea from the comics has a totally different personality from what I recall. Someone like Milla Jovovich, although more fitting, would have been a bit too expensive and it would have her in competition with Rick for the lead role which is antithetical to the storyline.
Andrea is not my favorite character, by any means, but I don't hate her or wish she didn't exist.
I don't know, maybe it is the writer's fault. I just don't like how her character has never really evolved into, a, well, character. She seems to be playing some sort of trope or cliche, or at least she has been for a while now. With this latest turn of events though (in yesterday's episode), I'm thinking if they do it right they can actually give her some depth, and make her scenes interesting.
Also, I was totally caught off guard as well Lucy. That entire scene was very well done, I'm very curious to see what happens next.