Disorder
These Communist sympathizers are learning the hard way that life is hard work, regardless of political beliefs. They want to share their space and their resources, but pretty soon, the group is nearly paralyzed by the disorder and laziness among their ranks. Their home is a mess, and through that concrete imagery, the reader sees the abstract disorder among them as well. They aren't strategic about their political views; they just hate authority and order.
Compromise
The slide into filth and laziness is only one kind of compromise in the novel. Alice is a petty thief. Although she pretends that her political views condone her behavior, she is clearly compromising because she wants money. She steals for the same reason anyone steals, but her willingness to steal is more dangerous, because she excuses her behavior. This tendency to compromise continues until she is willing to do more and more evil, turning a blind eye to her own actions. Eventually, she participates in a bombing, not because of true belief, but because of moral corruption and weakness.
Authority and resistance
The police are the bad guys as far as Number 43 are concerned. The police are the concrete representation of the abstract thing they resent most: authority. Notice that these Communists (if they even deserve that title) are basically angry rebels who are mad at their parents. Alice antagonizes her parents with personal vitriol, and her relationship to authority is clearly part of her developing political opinion.
Terrorism and bombs
Eventually, the novel makes good on its title's promise. When the terrorist attacks begin, the reader is asked by the title to determine for themselves if the attacks are truly justified and Good or if they are evil and morally compromised. Clearly, the novel's depiction of violence demands scrutiny. These kids are punks who end up as murderers because they are complicit with deep hatred and anger within themselves, so when organizations approach them with terroristic intentions, they wrongly believe that they have to do what they are told. Notice that they are terrorists because their aversion for authority left them perfectly accepting of authority of a different kind.
