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 Everyman: 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.
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.