At last the story is going somewhere. The previous 2 chapters were setting things up. Here we get Tobit praying to God for death. And the character of Sarah is introduced. Her tragic story is that she has been married 7 times but before she could consummate any of them the demon Asmodeus killed each of her husbands. A maid accuses her of killing her husbands and in despair Sarah intends to hang herself. But then, to spare her father dishonor, she prays to God for death. The prayers are heard and the angel Raphael is dispatched, not to kill Tobit and Sarah, but to solve their problems.
The parallels between Tobit and Sarah extend to unfair accusations triggering their death wishes. Tobit actually quotes Jonah in saying it is better that he die rather than live. Unlike Tobit, Sarah doesn't confess to any sins. And she gives God an 'out' should her death not be a pleasing option to him. Notice that she prays with outstretched arms, the ancient posture of prayer which is once again becoming popular.
The notes in the Harper Collins Study Bible suggest that Asmodeus is a folkloric demon lover who kills Sarah's husbands because he sees them as rivals. Yet he does not (or cannot) consummate his lust for her. The Interpreter's One Volume Commentary suggests that Asmodeus might be a Persian demon but could also be a Greek version of the Hebrew word for "destroyer." By the way Raphael means "God heals" which is his whole mission in this story.