When she was first learning magic, Willow had a plan. If she could just get enough power, she could make sure no one had to get hurt. She could make sure that bad things wouldn't happen, or at least that they wouldn't happen when she was there to prevent it.
Things rarely turn out exactly how we plan them. What Giles had failed to mention, or possibly didn't realize, was that there comes a time when you have so much power that you begin to really see the big picture. When you were just fighting the devil in the details, you could afford to see everything in black and white; us and them. When you started fighting the war on a grand scale, you quickly realized that it wasn't really about what you thought it was about.
It wasn't about good and evil. It was about balance. The whole 'no light without dark, no good without evil' line they sometimes fed the college girls at the spice-rack coven meetings wasn't just a line. It was a truth that was easy to forget when you were in the good times, and so very hard to accept when you were stuck in the bad times. It was a truth, however, that was painfully clear when you were forced to sit back and see everything happening at once.
She could see the possibilities unfurl as an over-eager disciple of an ancient demon became infatuated with a battle-tested genius who was struggling hard to do right in a world full of temptation. She could feel the guilt in her own gut that would one day slice through the Badlands warrior turned slick lawyer as he signed a coffin through so that he could hold onto what he had been given.
She had the power to stop all of what this would bring. She could easily have saved these people from the pain of losing someone they all loved. But she could see what that loss would bring; the ancient god that would rise in the shell of their dear friend. She could see how necessary that loss was to a much needed win in the battle against the dark.
It was an epic game of chess, and sometimes it wasn't just the pawns that got sacrificed. So she stood by and let Wolfram and Hart take another queen in their game against Angel and his friends. She even stepped in and ensured that Fred couldn't resist the urge to touch the gem that would seal her fate. She killed a woman who shouldn't have had to die, who had already given so much of herself.
If that was all she had done, if being a player for the side of chaos and destruction was what she chose, she knew another would rise in power to stand opposite her. Balance was the name of the game, after all. But Willow had never been really good at being just anything. So, as an ancient demon burned its way into the world through the dying body of a good woman, Willow reached down and cradled a soul that she would one day return to that body. Not to displace a lost god trapped in a limited world, but to guide it to a better path. Balance.
She had heard about the two wolves fighting inside everyone and how the one you fed was the one who won. She wasn't trying to be a rebel, but she wasn't very at good deliberately killing anything, even if it was just an allegory meant to teach a lesson. So she fed both and hoped they learned to lay down next to each other and be content.