Jesus taught his disciples to pray, "Your will be done on earth as it is in heaven." It seems that telling Peter that he's in essence Satanic seems quite harsh. After all is it bad to not want your friend to die and be willing to keep that from happening?

Well this time is is bad. If Peter's will be done he and us would still be in our sins and be judged and punished accordingly.

This is exactly why it is not good enough to have our own personal good intentions. Our best intentions are a mess when it thwarts the good and perfect and lovely will of God.

Matthew 16:21-27