or to summarize
You have a girl who was tortured and traumatized for years, wasn't able to express her own emotions, when she finally did have a crack in her facade some darkness spilled out which she couldn't deal with since she wasn't used to having emotions anymore, however that darkness didn't mean she was evil, just human. Sometimes victims have trouble becoming the monsters they feared growing up as well, there's a bit of that symbolism. But at the end she rejects it all, when her sister tells her that she loves her and Sakura finally accepts herself and chooses to be human again.
She had nothing to do with the killings in town, that was Angra in the womb simply seeking food within town. At three points in the novel Zouken and Kotomine say that Sakura had no control of Shadow's actions in her sleep. As soon as she did have any control the killings stopped. HF is about the concept of an innocent monster, a monster who births evil not out of any malice or evil from its actions, but because it exists. However within the core of that is still a victim that needs saving as well. This is a concept seem a lot in TM. Such as Akiha at the end of Kohaku route, Sacchin, Kohaku herself, any of the Alter Egos, or almost any monster character in TM.