Shuu's real intention of taking care of you when you mysteriously appeared in God Eden was because he wanted to sacrifice you to his God. He was so obsessed of getting his dead sister back to life that you had to be the victim.
But those things changed.
He had fallen so madly in love with you, and you, too, that you were willing to give away your virginity. He stopped thinking about bringing his sister back to life… because he wanted to live forever with you.
But you misunderstood the situation and thought he was really going to murder you for his God. You tried to break free from his captivity, but you accidentally fell into a chasm and hit your head against large-sized and pointy rocks that you stayed in a comma state for more than a month. Your physical wounds healed in time, but you suffered from a severe memory loss the moment you woke up from your 'sleep', which seemed like the deepest sleep ever.
He was unable to accept the fact that you had forgotten everything about him, including the relationship and the memories of the days you've shared with him. That made him suddenly feel inclined to return to his initial intention of saving you, which is, yes, to exchange your soul with his long-gone sister's.
Shuu also considered you as a pure soul who had never committed sins (because he believed you couldn't tell the God that you lost your virginity because you couldn't remember any details about your life), so he thought you would be the most gratifying sacrifice for his God. His resolve to get his sister back was set ablaze again and he pushed you off the cliff, leaving you to die in the pit.
And yes, in the end of the story, everything went black because you've already died