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Why did Beowulf feel obligated to avenge Grendel?

Read the excerpt from act 1 of a doll's house. helmer: nora! [goes up to her and takes her playfully by the ear.] the same little featherhead! suppose, now, that i borrowed fifty pounds today, and you spent it all in the christmas week, and then on new year's eve a slate fell on my head and killed me, and— nora: [putting her hands over his mouth]. oh! don't say such horrid things. helmer: still, suppose that happened, —what then? nora: if that were to happen, i don't suppose i should care whether i owed money or not. helmer: yes, but what about the people who had lent it? nora: they? who would bother about them? i should not know who they were. helmer: that is like a woman! but seriously, nora, you know what i think about that. no debt, no borrowing. there can be no freedom or beauty about a home life that depends on borrowing and debt. we two have kept bravely on the straight road so far, and we will go on the same way for the short time longer that there need be any struggle. nora: [moving towards the stove]. as you , torvald. how does the interaction between helmer and nora advance the plot? nora realizes that helmer will completely disapprove of her having borrowed money, so she has to continue to keep it a secret from him. nora realizes that she and helmer have the same ideas about financial issues, and the conversation brings them closer together later in the play. helmer realizes that nora is more responsible with money than he originally thought, and he trusts her more with finances later in the play. nora realizes that helmer knows a lot more about borrowing and lending, and she will seek his input later when she needs it.