I wonder if Kisimoto really should have have had Garra reform. Don't get me wrong, I liked seeing all the new sand jutsus, and the parallel betwen him, the only other Jinchurrikii that Naruto knew becoming a kage which is Naruto's goal. However, this goes completly against Garra's charchter is a psychopath, and psychopaths (along with Sex offenders, but that is another story) are incapable of reforming even with psychiatric treatment.
Signs that Garra is a psychsociopath during the Chunin Exam saga are clear cut. He is bloodthirsty, and kills for no reason. Finally, he is unable to control his bloodlust and is unable to hold himself back. Just about all of the other villans in the series have REASONS, however depraved, for their murders, and killings. Garra, however, does not. Basically, in reality Orochimaru (provided all off those jokes about him being a pedophile aren't true *LOL*) would stand a better chance of reforming then Garra.
Don't get me wrong, psychopathy can be born out of trauma in one's past, particularly during early childhood(i.e., Garra), istead of being (supposedly?) born this way. However, once they change into a psychopath, there is no turning back.
I basically chock the reason for this MINOR goof up to a incomplete understanding of psycopathy on Kishimoto-san's part.