Honor killing wouldn't surprise me. Happens here in the US more often than you think.
IMO, as someone born in the Middle East and someone familiar with similar cultural beliefs/mentalities, what Faiza was doing in terms of fighting for custody was very dangerous. As I'm sure many already know, divorce is looked down upon, and can even be disallowed. In some countries, it has to be granted by Islamic courts, in which all the judges are male. Of course, this isn't always the case, and is slowly changing, especially in more "liberal" countries.
However, if a woman does get a divorce, child custody is a whole other battle. On more than one occasion, I've seen women threatened for trying to take the children. It might be similar in Western cultures, but I'm not sure if it's to the same extent.
In regard to Faiza - her husband wanted full custody and was making her out to be incompetent, and mentally unwell, in an attempt to delegitimize her and her ability to be a parent. He started doing so from the time their daughter was born (literally while still in the hospital, after giving birth, according to Faiza's brother). She had her daughter at the time of her disappearance and was actively fighting to keep her. I would not be surprised if it had something to do with her disappearance.
Again, this is just my opinion, based on my experiences and what I know.