Merlyn was part of the league of assassins, and knows Sara was a member, who knows what he learned during that time..I just have to imagine they are going to try and put Ollie into a hard spot..and making his half-sister kill one of the girls he loved, would set his sister up for a pretty bad ash story line.
Komodo..Merlyn..and Ras would only set it all up for a fairly basic story, I could obviously be wrong, I just think everyone is ruling her out for little reason..merlyn won't be the one who killed her, I doubt they kill a fan favorite from a nobody like Komodo, and the only other option is Ras, who is definitely a possibility, but I have to imagine she would kneel or show a much different response if it was him.
I guess it could be Nyssa as well, but I have big doubts.