Was going to suggest this.
Everyone shits on the last third of Sunshine saying it's goofy that Pinbacker becoming a slasher villain ruins the movie. Except, Pinbacker's madness is at the core of what the entire film is about.
The protagonists are on a quest to restart the Sun. They are achieving this through human brilliance, courage, and ingenuity. Except along the way they give in to human fallibilities, the most egregious being mysticism/despair.
The one guy who never gives up and always keeps his mind on the goal is Capa. He represents humans overcoming our problems through smarts, science, and caring for each other. Pinbacker represents fanaticism and religion and all the silly nonsense that has held humanity back. He rambles on about being the last man left alone with God. How it's God's will for the human race to die. And of course, the first god was always the Sun. And of course, what do religious extremists always resort to? Violence. Pinbacker, who used to be a man of science himself, has given in to mysticism and despair. He shows up all blurry and warped because it's the outward manifestation of his madness. It's almost like he has been infected by the Sun (i.e. God).
Anyway, I love that film. Just think it's quite misunderstood.