The main subject of The Grass Crown is Marius and Sulla's growing antagonism and its (at least temporary) destruction of the Roman Republic. When the story opens, Marius' career seems to be in decline, while Sulla's has not taken off yet. Marius is popular as a war hero, but but his popularity, at least with the Senate, grows in proportion to his apparent decline. They love him at a safe distance only. Both men on separate occasions confront Mithridates of Pontus, an eastern despot seeking to expand his power and behaving like an eastern despot, i.e., regularly and arbitrarily killing anyone from his closest and most loyal advisers to his family members and making the general point that Rome, for all its faults, is a high trust society by comparison. Both men are able to intimidate Mithridates and keep him from growing so big as to be a threat to Rome. Plutarch, by the way, accuses Marius of deliberately seeking to stir up war with Mithridates to revive his lagging career. McCullough does not appear to take that accusation seriously.
Sulla also reminds us that he is the villain by poisoning Quintus Caecilius Metellus Numidicus, a conservative politician who was Marius' arch-enemy and went into exile rather than take an oath to uphold Marius' distribution of land to his soldiers.* There is no historical evidence of this whatever, although there were rumors that Numidicus was poisoned by Quintus Varius. However, Sulla's poisoning of Numidicus serves more purposes than just to remind us that he is evil. For one thing, it allows the author to contrast the devotion of Numidicus' son,Quintus Caecilius Metellus Pius to the treachery of Mithridates and his family and remind us the advantages of Rome. It also points up the changes in Sulla. Both Metelli are seen mostly from the viewpoint of Marius and Sulla. Numidicus, as Marius' chief rival, is shown in most unflattering terms. Marius extends this hostility to Pius as well, and initially Sulla does as well. But the innocent and unsuspecting Pius believes that Sulla was reconciled to his father and befriends him as the last witness to his father's life. Sulla is at first amused, but later comes to think of Pius as a "dear friend" and is even genuinely and unironically indignant at some wrongs done to his father (quite forgetting that he personally poisoned the man!). Pius is portrayed as a decent fellow, even though he remains devoted to Sulla after sensing what sort of man he is.
McCullough portrays Marius and Sulla as friends and allies, both supporting Drusus' efforts to extend citizenship to the Italians and desperately seeking to head off the civil war they know will follow if the Italians are denied. Plutarch, for what it is worth, makes no mention of either man championing the Italians and says that they had a growing rivalry over who would get credit for defeating Jugurtha of Numidia, that was prevented from boiling over only by the outbreak of the Social War, i.e., the revolt of the Italian allies (Socii) after Drusus was killed.
Plutarch portrays Marius as rather indifferent commander in the Social War, presumably due to increasing age and declining health and says that he worked hard to get back into shape afterward. McCullough portrays Marius as not being in any way malicious toward Sulla, but appallingly insensitive. While Sulla does us utmost to impress Marius with his ability as a general, Marius is dismissive and assures Sulla that while, yes, he is a good general, Marius is an inspired general. When Sulla asks what if anything happens to Marius, Marius says that will depend on whether Rome can get by with a good general. Sulla quite understandably wants to punch him in the gut. Shortly afterward, Marius suffers a stroke and is carried home, paralyzed on one side. Sulla proves himself an inspired general and wins the grass crown (hence the title), Rome's highest military honor given only to a commander who saves a legion or raises the siege from a city. Not even Marius had ever won the grass crown. And while he acknowledges that he was wrong and that Sulla was in inspired general after all, he does not acknowledge that Sulla is a better general than he was, so Sulla remains resentful. The author also strongly hints that Marius' stroke has caused disturbing personality changes. It is at this point that she shows him hiring a hitman. McCullough also gives an appalling and presumably accurate portrait of the atrocities committed on both sides during the war and the hardship it caused.
In the end, Rome extended citizenship to all Italian allies not in revolt. In the novel, at least, this utterly disgusts Sulla, who sees that it is merely the prelude to extending citizenship to all Italians. And, indeed, each member of Rome's ruling class who is not a Roman of the Romans sees the opportunity for clients if the people of his region are admitted and pushes for their admission. (Drusus did his cause serious harm by claiming all newly enfranchised Italians as clients. If he had pointed out all the prospects for clients, he would have gotten a sizeable constituency to support him). Sulla, as war hero, is elected consul. The rivalry between him and Marius will soon break out into the open.
*McCullough really should have included a section on poisons in her glossary. She is a neuroscientist who took care to describe the symptoms of each poison in enough detail to make clear that she had a specific one in mind. Sulla poisons Numidicus with "a nasty brew decocted from peach seeds," i.e. cyanide. The symptoms of cyanide poisoning are very similar to acute respiratory failure -- dizziness, choking, and gasping for air. The main difference is that in acute respiratory failure, the blood cannot become oxygenated, so the victim turns blue. In cyanide poisoning, the cells cannot get oxygen out of the blood, so the victim turns red. One of the attending doctors shows the strange mixture of incompetence and competence of the time and proceeds to bleed Numidicus and is surprised that the blood comes out of his vein bright red. This leads him to suspect poison, though not to name a particular suspect. That is a bit problematic. The trouble with cyanide (from the perspective of the poisoner) is that it is so fast-acting that once poison is suspected, there is usually not much room for doubt whodunnit. Who could have poisoned Numidicus? Probably the guy he was drinking with all afternoon before he got sick.