Oh, the ending. Spoiler alert: make sure you fill up with happy before you get to the end of this book.
Jim's fate may not come as a surprise to you. After all, his wound doesn't seem to be healing, anyway, and Steinbeck gets heavy on the light-burning-out symbolism. However, the exact method of Jim's death truly shocks, as it's meant to.
Steinbeck has Jim maimed in such a way that we have to think of the meaning of his fatal wound. Seriously, Jim literally loses his face, the body part most closely connected with personal identity. This is no coincidence, since Jim has willingly bartered his life and his individuality to become one with "the cause."
By giving up his personal identity to become a Party man, Jim has also given up a major part of his humanity—hence the scariness of his behavior and the phenomenal power he feels coursing through him. He has become something beyond human, so such effacement seems appropriate. (Sorry. We had to do it.)