your middle statements are completely untrue. nothing in the bible says you must inflict pain on yourself. those monks made up their own religion apart from the truth of God's word.
before a holy and sinless God we ARE unworthy and sinners. before a holy and sinless God we are worthy of nothing but death. but because God loved us and valued a relationship with us so highly sent his son as a sacrifice for the sins of man in order to restore the relationship God had with man before the sin of adam.
on top of all this your middle statements have absolutely nothing to do with "love your enemy". you've apparently moved on to something else.
as for your question, Jesus sacrificed himself on the cross because this was the will of the Father and he wasnt going to disobey his Father because thats what adam did and man was cursed and separated from God because of it.