YA; I read it sometime between approximately 2007-2013. A teen girl discovers that her dad killed a man (possibly his girlfriend's husband).

[Savannah]

There is a scene where the dad removes his scarf and there are dozens of scars on his neck (presumably hickeys).

*SPOILER AHEAD*

I think that the final scene is either the body being found in the dad's trunk, or him being taken away in a police car (or perhaps a combination of the two).