Does it make sense that Abigail. . .

I think it is to show that, even as a vampire, she still had a shred of humanity.

I was fine with it as they both worked together against frank. The pinky signified a bargain.

The alternatives would be
Abigail kills Joey, people are upset
Joey kills Abigail, people get upset because Abigail killed everyone else... even in a group attack.

This seemed like the best calculated ending.

I didn't see it as her turning "good". She took a liking to one of her planned victims, and the two of them had to become a team to survive at the end. They bonded over that.

I don't think she really changed her ways too much, though.

lots of things didn't make a whole lot of sense
Those shutters should have been made of metal - if it was possible to break one up with a wooden stick and get light beams into the vamp, you got a real planning issues in your playground lol

Also - you know how that black dude got taken out? It was done in a second - without a sound. At no point did that little girl display that sort of ability during the actual fights. So, how did she do it? How did he get that huge wound on his face and neck without a sound and within a second? It made me think that there was an alien-like creature out there, but alas. Anyways, plenty of inconsistent BS in the film - it's a fun film, but it had those sorts of issues all over.