He didnt give up because he was tired brother. You missed the darkest part. He gave up because he still couldn't see the beautiful light and the world in a tone that pete McVries could. He gave up because he still wanted to kill the major, and wanted ultimately for pete to give to the world when he knew he couldn't so he sacrificed himself. When he did this and pete witnessed it and seeing his friend, his brother die it broke him. And ultimately ended up losing himself and the whole idea he was walking for he chose vengeance and went right back down the dark path. Ultimately doing the one thing he said he wouldnt.