I see chapter 3's couple as a pitiful future. One who will do anything to keep the one she loves by her side, knowing that the longer she continues to do this the more and more twisted her original feelings will become. It's the knowing that you can have everything about that person, except the one thing you're longing for the most, that type of emptiness really isn't worth what you get from the relationship.
And the other, who will atone for not being able to return the feelings that the other one has. She must believe it to be her greatest sin, great enough that she can throw away everything to repay it; her happiness, her future, even potentially her love, if ever she finds it.
This couple feels like they'll be together for a long time. Falling deeper into the abyss, knowing that they should end this pitiful, toxic relationship, but just can't extricate themselves from it.