That's one of the rules, and you can't avoid it. Even the work of a professed atheist has to follow the rules, if its to make sense, let alone be compelling. I don't mean the rules of fiction; a good writer can break them at times. I mean the rules of the world - the rules of God. As God is the author, only He may break those.
When a great evil is to be fought, there is almost always a need for sacrifice. That's true in our world, and it's true in the worlds we subcreate. Consider three of the great "battles" in the season finales of Buffy the Vampire Slayer. I put "battles" in quotes because in none of these cases was it the battle that won, in the end; it was the sacrifice that ultimately won the day.
Against the Master (a blasphemous name to Christian ears, and possibly to Doctor Who fans'), Buffy decides that all she can do is give her life. It is that seeming failure that brings the master vampire to the place of his defeat. It is that apparent end that sets things in motion for the final victory.
Giles reads a prophecy that this is going to happen - that Buffy will die in her efforts to defeat the Master. Why is his faith in Buffy shaken when he learns that she's going to die? He believes that death is final, and that when Buffy dies, that's it. How could he know that she'd live again, and that a great good would be worked from that great evil? Only by faith (the theological virtue, not the slayer). It doesn't come easy, and, in fact, we can't come by it on our own. Faith is a supernatural gift from God, and we cannot truly have it without it coming from Him.
Very early on, we see Buffy start to realize that "death is her gift". It takes her five seasons to learn what it means - giving of herself, dying to how she plans her life, dying to absolute dominion over yourself. She begins to recognize here that a sacrifice must be made and that she is the one to do it. She doesn't know it yet, but that's not resignation and despair. It's power.
Yes, death is our curse, the cost of the first sin, but at the same time, death is our gift. Sacrifice has power over evil. Evil is selfish and cannot understand or use the power of self-sacrifice. By giving of herself, Buffy accomplishes something that no weapon or spell or other action could accomplish. She finally uses a magic that truly works, saving the world by dying to it. She accepts death at the end of season one and more definitely in season five, then finally allegorically in season seven, by descending into Hell(mouth).
It is Spike's sacrifice, in the end, that closes that gate of hell. (Here we have another curious choice of name. Buffy and the potentials were saved be one Spike; we were all saved by three.) Again we see the selflessness that evil cannot understand. Angel receives that necklace, and evil could never imagine him giving it away to Buffy, nor Buffy naming Spike her champion. Power is meant to be used, not given, isn't it? It is for the strong, not the weak, isn't it?
Spike sought to gain back his soul, and throughout the season he cursed it or questioned in. At the end, he understood. He got his answer. "I can feel it, Buffy... My Soul. It's really there."
The words of Job would fit well on Spike's lips:
"I have dealt with great things that I do not understand; things too wonderful for me, which I cannot know. I had heard of you by word of mouth, but now my eye has seen you. Therefore I disown what I have said, and repent in dust and ashes." (Job 42:3-6)
* You can read it, though: Ordinatio Sacerdotalis. Also, you can read Dr. Lowery and/or listen to Dr. Kreeft treat the subject effectively.