The movie could have been, and should have been, a five star effort, but the ending erased any possibility of that ever happening. In a span of five minutes, the movie went from a classic to a predictable disappointment.
Why, oh why, did Mud not die at the end? Clearly, and without argument, the main character in the flick was Ellis. It was a sufficiently obvious that the kid had learned a life-changing lesson when he stepped out of his dad's pickup truck and saw a young lady wink at him in front of his mom's apartment: "there are many more fish in the River. Ain' no need for me to end up dead, like Mud, because of one woman".
Instead, everyone had to live happily ever after; even the repugnant, mindless Mud. Why make a movie deep and meaningful when you can make it shallow and predictable?