All of his thoughts were churning into one, making everything now a very terrible thing at first, his chest hurt, his heart ached and all of that because of his deep longing for Grace. Or even this is God's remission of sins for Nicholas, when he has no power to do anything and can only regret all things like this when the person he loves cannot be found and he ends up dying in a state of deep longing. Yes, that's how Nicholas feels now, so Bowman is now his very real target, before he takes revenge on everything so perfectly, after he takes revenge he will make sure that Bowman will pay dearly for everything that happened to make him commit the biggest sin, namely by killed Grace's parents.