I think that a diary entry from the frog's point of view that details some of his characteristics and traits would have to talk about how he enjoys the position of power in the bog as being the source of all music.  The diary entry would have to reflect his enjoyment of this monopoly of power and the willingness to maintain the Status Quo as it benefits him.  There is little in way of innovative sharing of the spotlight or a collaborative sensibility in the frog, as he recognizes the threat posed by the nightingale the moment she starts singing and how the other animals react to it.  I think it might be good to invoke a bit of the Iago- like traits fromOthelloin the diary entry.  The frog should make it clear that he can benefit by assuming the role of the mentor and coach to the unwitting nightingale.  I don't think that the frog is absent from intent in this and I think that bringing it out in the diary entry would make sense.  The ending of the diary entry, or the last entry should not necessarily dwell on how the frog wiped out the voice of the nightingale, or even that he was able to neutralize the competition, but rather focus on how there is some level of joy in the frog's reinstatement of the Status Quo.  I think that it is important to bring out that the frog is not a sadistic beast or the like.  Rather, he recognizes how threats to his own condition have to be eliminated in order to consolidate his own control and state of being in the bog.  This might be a good approach to take in writing a diary entry from the point of view of the frog.