Though I would recommend watching the whole movie, there are parts you can skip, if you binge watched the whole anime, unlike those who watched it weekly when it was aired. The first 10 minutes and the last 15-20 minutes have to be watched, since they have new scenes that convey the finale of the anime much better that the ending of season 2.
The movie is a must, if the first and second seasons made you feel miserable. It just means that you didn't pay clear attention to the important details that were mentioned in the anime. I am one of those who felt incomplete, and the movie was just the thing to make me appreciate the anime to the fullest.
