How would you justify the frog's actions in the poem "The Frog and the Nightingale" by Vikram Seth?

Before the nightingale arrives in the bog, the frog is the only voice in "The Frog and the Nightingale" by Vikram Seth. Nothing "Stilled the frog's determination / To display his heart's elation." When the nightingale arrives, the frog is immediately replaced by her beautiful voice. In order to manipulate the nightingale into wearing out her voice (and, sadly, dying), the frog uses flattery. While this strategy may or may not be justified, it is certainly very savvy of the frog to praise the nightingale so she will do what he wants. The nightingale is "greatly flattered and impressed" that the frog praises her, so she allows him to change her song. The nightingale sings longer than she should, as she's impressed "with a breathless, titled crowd." The crowd that gathers to hear her sing is from the nobility, so she keeps singing, flattered by the attention of important people, even when she shouldn't. When the nightingale dies from singing too much, the frog says she was "Far too prone to influence. / Well, poor bird - she should have known / That your song must be your own." Even though the frog's actions were cruel, the nightingale fell prey to being his victim because she was vain. She should have just accepted that her song was short but sweet.