a n g e l i c o r p s e
Because Jecht became Sin. Somehow Sin needed a host and picked Jecht for some odd reason which he wanted Tidus to kill him and put him to rest because he knew he wasn't a great father role model to him all those years.
jecht became the final aeon for yuna's father and yu yevon alwasy pick the final aeon to become the next sin after the 10 years of the calm. its says it in the story. but its a sad story becuase at the end of the game jecht had feelings for his son. Also at the end of the game Jecht said that he was holding sin's true power back, so his son could have a chance to kill him as he was still human.