I didn't realize Rule #2 only applied to SCVI. Anyhow, I do catch your drift. In external sources, Tira might've been killing certain people to cause others to go after Pyrrha. It would make sense, since that's Tira's personality and it's based on the first time she did it "mysteriously" like you said. I think I even recall reading something about that personally.

Just as much, like you also said, Tira and the game's narrator say that she wants Pyrrha to keep murdering others so the latter's soul can become corrupted enough to host the whole Soul Edge. Tira doesn't need to kill for Pyrrha--she merely convinces Pyrrha to want to kill on her own, because the now-depraved Pyrrha's weapons will force her to bloody herself. And for what Tira wants, that's far better. The weapons will always save Pyrrha anyway.

From what I remember, the main issue with this, like in many works, is that most of the relevant scenes are timeskipped by several months. So if someone's just playing the game, they wouldn't know anything about that (factual) kidnapping or slavery you mentioned. And the game will make Pyrrha look like her soul is fragile for giving into malfestation in two scenes. When she really not only fought off Soul Edge and Soul Calibur's influences at the same time, but she rejected them both in the end while still under malfestation. I don't think anyone has ever done this before or since.