Assuming that Madaras story is true...(recent spoilers included)
Why did Itachi have to kill his girlfriend?
On the one hand we got some girl who Itachi was close to and for whatever reason had to be killed? I doubt that some girl Itachi met and loved would join up with the rest of the Uchiha clan (which has no relation to her besides Itachi) in order to take over the village.
So there is no logical reason for her to be killed. A friend of mine said something about Itachi not wanting witnesses. Eh. He had to maintain the look of a criminal anyway so I think it doesn't matter but even if he wanted to avoid witnesses he could've used genjutsu and saved her life. I'm sure Itachi wouldn't kill someone who didn't need to die.
The only possible explanation as to why Itachi would kill his girlfriend..and this is the real point of the post...
Inter-clan breeding. For the linguistically challenged I'm saying that Itachi was dating someone in the family. And not just Itachi but for a strong clan it would make sense that they would do it. To stay strong you need the Uchiha blood to stay strong and it makes sense that the sharingan would be most likely to appear if both of your parents have it.
Less Sharingan and less Uchiha blood would make the clan weaker right?
If I'm right then Itachi killed his girlfriend because she was of Uchiha blood.
Or I might be missing something. Please prove me wrong because I don't want to think of Itachi every time I watch Jerry Springer.