The Count is a villain that the writers are evidently quite fond of since he's still going after two episodes. The Count's first appearance led to Thea's car accident and subsequent community service wherein she met Roy from The Glades. Oliver hasn't forgiven The Count for bringing a drug into Starling City that nearly cost his sister her life. The drug resurfaces and gets a girl killed on a crowded street. The death brings attention to the club because Tommy's books suggest some illegal business is happening in a secret room to get more people, and their money, into the club. The episode uses an old villain and his weapon to tell a personal story of revenge and mercy for Oliver while also using the same villain and his weapon to tell another personal story between two friends who are growing apart. Indeed, it is about "Unfinished Business."
I'm convinced Oliver's friendship with Tommy will follow the course of Peter's friendship with Harry in Spiderman, so it's important to show where the friends were and where they are now. Tommy's depth as a character has been shown in his complicated relationship with his father, Malcolm. Tommy and Oliver have been apart for many episodes this season. Tommy's relationship with Laurel temporarily put a wedge between the friends. Oliver's reveal to Tommy about his other life was significant, and Tommy reacted poorly to the revelation. Tommy and Oliver simply aren't as close as they once were; and the glory days aren't coming back. The disintegration of their friendship is happening but it's not that surprising; the two haven't really had a strong friendship since Oliver's return. Oliver's closest relationship is with the people who he 'works' with, i.e. Diggle and Felicity. His closeness with Laurel isn't being felt as strongly as the writers would like, and his interactions with Thea and Moira are too few and far between to have a lasting impact. The show successfully made Oliver an island unto itself, with two close companions, as it was on the island with Shadow and Slade.
Since Tommy and Oliver's friendship is lacking in the present, Arrow resorts to the past to get something--anything--out of its disintegration. Oliver doesn't defend Tommy against accusations about selling vertigo out of the club, because he remembers a time when Tommy hung with hard people. Tommy's honest and reformed, though, and his frustrations with Oliver are about Oliver's reluctance to accept his honest transformation. The police suspect Tommy because of Oliver's vigilante work. What raises red flags involve Oliver's vigilantism, and Tommy's forced into covering his friend's ass. The friends reach a mutual truth in the end of the episode after misunderstandings and hurt feelings--neither knows the other well anymore. Oliver changed on the island, but so did Tommy. Tommy won't cover Oliver's ass, so he quits Verdant and asks his father for a job.