Yuuchi didn't actually do anything wrong. Though she was a minor, the girl was the manipulator. The girl was the one to pressure him so, not the other way around, and her shitty mother and abusive upbringing is no fault of his. What has he to feel so guilty of?
Stop acting like there's a meaning to life. Most of us could've never been born and humanity wouldn't have suffered for it. There is no meaning. That's not a bad thing. Just enjoy life and stop hamfisting meaning where it doesn't exist. Sure, you can have your own subjective purpose for living...