why did sideshow bob really wants to kill bart simpson?

It was because Bart was the one who managed to foil Bob's plan when he wanted to frame Krusty and ruin his career. Ever since that day, he wanted to have his revenge on Bart, but no matter what he did, Bart always managed to beat him, so I guess he made it his life's work, to not only ruin Bart's life but his whole family as well.