Catch-22 Dilemma: Working for both bosses presents the problem of having the other side likely killing him for succeeding. Subverted in that Slevin is revealed to be playing them both against each other in order to avenge his parents.
The Chessmaster: Surprisingly, he plays both The Rabbi and The Boss like a fiddle.
Deadpan Snarker: In part due to his condition, Slevin has no problem snarking even in the face of imminent danger.
The Everyman: Unlike most of the other characters, Slevin is relatively down-to-earth and just normal. Subverted as it's exactly what he wants others to see him as.
Flirting Under Fire: Slevin and Lindsey build up most of their romance in this manner. Despite the fact that Slevin is on the hit list of two warring gangs, he and Lindsey find time to flirt, go to dinner (where Slevin is able to shadow a man he has been told to kill), and spend a night together.
Laser-Guided Tykebomb: Raised from childhood by Goodkat to one day enact a brutal revenge on those who wronged him.
Meaningful Name: Slevin is the name of the horse race—Lucky Number Slevin. Kelevra means "bad dog", referencing his relationship to Goodkat, as well as Brikowski.
Revenge by Proxy: Murders both The Rabbi and Boss's sons for this exact reason. Their deaths are simply a means to an end in the long run.
The Scapegoat: Both the Rabbi and the Boss are quite happy to use him as one to get to the other. Slevin is not only fully aware of this, he's counting on it.
Shirtless Scene: Spends a good portion of screen time in nothing but a towel.
Trauma Conga Line: He informs Lindsey about how he came to be in his current predicament. He lost his job, comes home to find his girlfriend cheating on him and then gets mugged. Then a bunch of gangsters show up, mistake him for someone who owes them a lot of money and it gets worse from there. It turns out the whole "bad day" story was a fabrication and he deliberately engineered the case of mistaken identity as well.
Hitman with a Heart: It turns out that when Slevin was a child, he refused to kill him because he Wouldn't Hurt a Child. After realising that Slevin helped Lindsey fake her death, he lets her live and leave with him.
Faux Affably Evil: He sounds pretty calm and restrained, but he is always making coy, veiled threats with his words.
The Killer Becomes the Killed: The Boss and The Rabbi murder Max and most of his family. Several years later, Max's son destroys everything he cared for and kills him (plus the Rabbi) the exact same way that he did.
Laser-Guided Karma: Slevin kills his son and then suffocates him exactly the same way The Boss did to his father.
Face Death with Dignity: While the Boss gasps and despairs as he dies, The Rabbi merely gives a resigned stare to the horizon.
The Killer Becomes the Killed: The Boss and The Rabbi murder Max and most of his family. Several years later, Max's son destroys everything they cared for and kills them the exact same way that they killed Max.
Laser-Guided Karma: Slevin kills his son and then suffocates him exactly the same way The Rabbi did to his father.
No Honor Among Thieves: Tried to murder his former criminal partner the Boss. Why? Only one person can be the boss. Since Anthony was called The Boss most assumed he was the one ultimately calling the shots. That didn't sit well with The Rabbi. So he felt he had to remove the "confusion" over who was running the show.