1. Frog was in a want of revenge because Nightingale seemed a tough competitor for him. According to him, all his popularity was taken by the Nightingale. So to take revenge he put the Nightingale all day in rain , passing bad comments on him. So he seems a clever frog .
2. He wanted to ensure that no creatures in the forest like the Nightingale's song. So he lure the Nightingale to sing even though her voice was getting harsh. This shows his Shameless behavior.
3. At last frog wanted to ensure to finish his competitor. So he get the Nightingale to the death. This shows his Pitiless Behavior.