The motif of political corruption
Not only is the nation-state of Jamaica completely overrun by corrupt politicians who enforce their rule through gang terrorism, also, the United States CIA (in the book) seems to have played a serious role in Marley's persecution, Alex believes. Also, the gangs themselves are riddled with corruption.
For a quick example of the gang's corruption, look at Josey in prison for several first-degree murders. He claims to still be in control, through his son Benji (who ends up shot and dead). But before Benji dies, Doctor Love comes to visit, and tells Josey that Eubie hired the hit on Weeper, and Josey attacks him. This constant self-betrayal is what defines gang life, and it defines the political reality of Jamaica at the time.
The motif of organized crime
In this novel, we see how cocaine made it to certain neighborhoods of New York City in such vast quantities. The Jamaican gangs work closely with the government, so Jamaican gangsters (mainly Josey) travel to New York City to sell the cocaine they sell in Jamaica, abroad. They get their drugs and guns from the Medelliín Cartel, a Colombian drug empire.
By the time Josey is in New York, we see how brutal the supply chain is in criminal organizations: he learns his dealers are using the drugs they're supposed to be selling, and he kills them all, not knowing that American police actually arrest people who do that (because in Jamaica, the police are the most corrupt entity of all). In prison, he claims to still be a kingpin, but it is clear that his empire has been overtaken by Eubie. This is a complete picture of the basic supply chain from Colombian cocaine farms, all the way to the cocaine epidemic in New York in the '80s.
The title story as a symbol
In this real novel, A Brief History of Seven Killings, a reporter for the Rolling Stone tells of his time in Jamaica, and he tells the truth about Bob Marley's legacy there, and how he came to represent hope and love to a broken nation. He tells all in an essay for the New Yorker called, "A Brief History of Seven Killings," the same title as the book, making the book a symbol.
Ultimately, his publication symbolizes his "sharing the gospel" of Bob Marley with a new nation. This evangelistic understanding makes sense, especially since the next chapters portray him being beaten and tortured into changing recanting parts of his story. In other words, Marley is a prophet, and Alex is like his first martyr.
The messiah symbology
The folklore around Bob Marley has always been that he was a true prophet from God. The Rastafarians are Christian mystics who live in Jamaica and smoke marijuana when they pray. They worship Haile Salassie I from Ethiopia as a reincarnation of Jesus Christ, so Marley, as an avid Rastafarian, likely believed that Jesus Christ could reincarnate and come again.
This belief shaped his music and his thought life, and he formed a personal philosophy around these ideas. Before long, he was the most important man in his entire nation, because he preached love and peace. After years of terrorism and intimidation, Marley died, and his legacy grew, partly because of Alex. If Marley is the "Christ character" of this story, Alex is like Paul, the evangelist prophet who takes the story to the foreigners (the Americans). As a messianic story, it checks all the boxes. The author's intent is unmistakeable; this is done to elevate Marley to a place of religious authority. In other words, humans ought to live according to the love that Marley preached.
The motif of horror and violence
That religion stuff might sound fluffy or nice, but it comes at a great cost. Marley's life is riddled with intimidation and persecution. The corruption of the government was so thorough that even the political parties were fighting against him, and using gang violence to do it. They attack his home, apparently with CIA intelligence (according to Alex the reporter). The police abduct Nina and almost rape her, and they murder Papa-Lo.
In America, Josey's violence is truly heinous. He murders many people in cold blood for dipping into his stash of drugs. In the hospital when Benji dies, the neighborhood forms an angry riot. In prison, Josey attacks Doctor Love for telling him that Eubie killed Weeper in a hit. These pictures of violence are designed to remind the reader that Jamaica has a very broken, violent, horror-filled past.
