Over the Falls
It seems like the whole point of the book is to find a way to take care of Thomas. After all, that's been the whole point of Dominick's life. He even promised his dying mother he'd take care of his brother, and breaking that promise would be like killing the poor woman all over again.
Dominick actually succeeds in freeing Thomas, but less than twenty-four hours later (and in the same freaking chapter) Thomas commits suicide. What was the point?
Dominick even wonders, "If your twin was dead, were you still a twin?" (42.19). The irony is that, un-twinned, Dominick is able to become more whole. He realizes that he can have his own identity separate from his twin; he stops seeing himself as a mirror image of Thomas and starts seeing himself for who he really is. Other people see it, too, like his ex-wife, Dessa, who remarries him.
Thomas's death is the first domino in a chain of events in the denouement that also leads to Ray telling Dominick who is real father is. It turns out Dominick is a descendant of a Wequonnoc Indian, making him a millionaire because of the new casino opening up. Wow—Thomas dying is the best thing that could have happened to Dominick. The moral of the story seems to be that if you have a twin, you'll be better off when they're dead. We know that much is true.