Loneliness
In Duffy's portrayal of a budding murderer, loneliness is part of a vicious cycle. The murderer's egomania, anger, and violence make him difficult to like, as evidenced by the radio host's hanging up on him. At the same time, the loneliness and isolation that result from this only increase his feelings of resentment and bitterness, which in turn cause him to act even more violent and angry. The speaker's loneliness isn't only of a social variety. Rather he seems to be cut off from society in every meaningful way. He implies that he found school uninteresting and hard, and he is unemployed. This radical alienation drives him to disconnect completely from reality.
Cruelty
While the decision to commit murder is clearly consequential for the speaker, it is far from his first violent act. Instead, he builds up to that violence, primarily by killing and torturing animals. He first kills a fly, and notes that this has been a habit of his since childhood. He then flushes a fish down the toilet. The deaths of his pet bird and cat are implied by his assertion that "there is nothing left to kill." These smaller acts of sadism seem to help him delay killing a person: he only actually picks up his murder weapon once he has killed his pets. Yet they also serve as a form of practice, desensitizing him to the act of killing. Thus Duffy frames violence as a spectrum, with senseless murder occupying the extreme end.
Fame and Infamy
The speaker decides to kill another person, not because he thinks of himself as sadistic or violent, but rather because he thinks of himself as almost god-like in his intelligence and talent. In other words, he feels slighted by the world's uninterest in him, and is convinced that he should be famous. For the speaker, however, the infamy he can achieve through murder is the shortest or only route to celebrity. Indeed, he appears not to distinguish at all between positive and negative varieties of celebrity. By displaying this thought process, Duffy interrogates popular portrayals of evil, suggesting that they in some ways valorize perpetrators and therefore push certain individuals towards violence.
