Jecht's intentions may have been good when he agreed to become Sin, but the important factor there is that Jecht is Sin. If Sin is the villain, then Jecht is the villain. I know Jecht didn't have complete control over Sin, but he at least had enough control to bring Tidus to Spira, as well as to provoke Tidus and his crew enough to kill him.
Provoking somebody to kill you is called being an antagonist. An antagonist, aka, a villain, is somebody who has a negative effect on other characters. This person doesn't necessarily have to be evil.
Also, the spoiler things are stupid. Final Fantasy X came out 13 years ago. It would be a spoiler to talk about it 13 years ago, before anybody had a chance to play through the game. Now it's just old news.
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