They were never evil... They are shinobi who must do what they are ordered to do. Now they are ordered to protect these people, so they do itIt's not that they turned good, they just decided it'd be a better idea to support Konoha thanthem
They also said in the Chunin exams arc that, in order to compensate for a reduced headcount, the Sand had to increase the quality of each individual shinobi.
The Sand aren't stronger than the leaf. It's actually quite the opposite. If you pay attention to the Chunnin exams arc it becomes very clear. Gaara and his siblings are the children of the 3rd Kazekage. Naturally they will be stronger.
Remember, the Sand shinobi we're seeing: Temari, Kankuro and Gaara are offspring of the most powerful clan in the hidden sand village. They're the late Kazekage's kids.
I would say that the Sand Shinobi are beyond the entry Chuunin level even though they did not pass the exam. For the most part they make the Leaf Gennin look like chumps.
it took forever for the leaf to try and "finish" their enemy, and it only took one ep. for the sand to finish them off. they sure kill with no hesitation.
i'm guessing,,, temari really hates green since she's from dessert.... hehehehe..!!! i have a feeling that shikamaru and temari might end up together.. i will not complain if its so..!!!hehehe