WALUIGI INSTEAD OF WALPURGISNOCT IS MAKING ME LOSE IT IN THE BEST WAY - Tumblr Posts

I'm pretty sure it's because the weight of her wish (the wish to end Walpurgisnoct, the most powerful witch made from several magical girls) overwhelmed her to the point her soul gem instantly (or shortly after wishing) darkened.

I think that's it (at least it's what I gathered from watching the scenes where Madoka becomes Gretchen)

It's mentioned that in every timeline Sayaka contracts and becomes a magical girl, she becomes Oktavia.

I think this says a lot about Sayaka's mental state. Even if Hitomi doesn't confess to Kyousuke, even if Sayaka and Kyousuke were to date, Sayaka would still become Oktavia.

Maybe it's a case of regretting what she wished for, or falling out of love with Kyousuke, or that her relationship with Kyousuke didn't work out how she had wanted it to. Regardless, something always goes wrong when she makes her wish; and I find that very fascinating.