On the surface it seems that Peter does not understand that Jesus is going to die. But in the next verse, Peter says that he will lay down his life for Jesus.
Ultimately, Peter did follow Jesus to a cross to die in the service of his king. He understood that Jesus was going to die. He simply could not realize that Jesus had it all under control and Peter's death (at that time) was not part of the plan.
Jesus did not need any help from Peter or anyone else. The only help Jesus requested from them was for them to stay awake and pray with Him, but even this one thing they could not do.