Its great writing because of how tragic the situation was.
You have this real good kid who has a bunch of dreams and only cares for his friends, someone who would never turn his back on comrades. This Kid dies a tragic death to save his friend who only turned his friend that day. In his last moments this kid gives his special eye to his friend and asks for one thing to keep the girl he loves safe.
After reading that part of Obito's story you cant help but say he was a great character and hero.
This is changed by the big reveal of a main villain in Naruto. Tobi is unmasked as Obito the great hero. But how?
Madara's influence was so great he turned the greatest kid into the most dark man.
Kishimoto really mapped out everything up till that moment of the reveal, you can really tell. The introduction of Obito's character in Kakashi Gaiden, which I thought was only to show how Kakashi had gotten his sharingan, but it was really to introduce a very important character later on.
The reveal as Obito:
+ Made the most sense so it fit perfectly with the story.
+ Made Madara out to an amazing villain. To be able to corrupt someone as good as Obito was really shows how bad he was.
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So Orochimaru was the main villain in part 1 but Kishimoto gives insight to the main villain of part 2 (for the most part) right at the end of Part 1.
It was a big moment in Naruto as well. I think Kishimoto wrote it very well.
I believe it even trended on twitter. #Tobi