Recently, I noted that the writers have been exploring the theme of sacrifice in the last few episodes:
First, we learn that Desmond used to be a monk, and was uneasy of Abraham's willingness to sacrifice Isaac on God's orders. In that episode, Desmond struggles with the issue of "sacrificing" Charlie-- his visions of the future caused him to see Charlie dying, and a rescue attempt. Desmond feared that he needed to let Charlie die to allow this scenario to evolve. In the end, he doesn't do it. He is unwilling to sacrifice an innocent for some unknown "destiny".
Second, Ben asks Locke to kill his father in some kind of ritual sacrifice as an initiation rite. In this, the Others seem almost demonic. Despite the fact that his father tried to kill him, and left him in a wheelchair, Locke cannot do it. Instead, he cons Sawyer into killing him. As we would say, this is formal cooperation in evil, and doesn't let Locke off the hook...
Third, Sun is pregnant and in a no-win scenario. If the child was conceived on the island, she will probably die, but she will know the father is Jin, her husband. If the child was conceived before they arrived on the island, the father was the guy she was having an affair with. Sun wants the first scenario to be true, for her husband's benefit, even though she would lose her own life. She is happy when this is confirmed.
Fourth, Desmond again presents Charlie with a dilemma. He sees him dying again, in the context of disarming some system that jammed signals from the island. If Charlie does it, he dies, but everybody can be rescued. But this time, Desmond tells Charlie. In a very moving episode, Charlie accepts this responsibility.
Very interesting discourse on the different forms of sacrifice!