Parallels between Slim and Ridgeman
Both were motivated by somewhat noble intentions, with similar circumstances provoking them.
Slim needed to get his mother and brother out a tough spot, as the mother was engaging in prostitution in a sleazy apartment. Ridgeman needed to get his daughter and handicapped wife out of that racist neighborhood, as the daughter faced harassment. Both, therefore, engaged in their actions for family. Neither were really hardened criminals. Both were sympathetic.
Both lost their close, best friends at the end (Vince Vaughn and Michael Jai White). They were both about to survive until Mel Gibson pointed a gun at Slim in the car. Even when they were driving to the warehouse, the people in car both say “possum ... refrigerator ... etc” as if the movie was trying to get us to notice the parallels. Also - when Ridgeman was in the black car and Slim was driving the white SUV, both of them pull the gun from out the glove compartments, almost in the same fashion.
These parallels are notable because of the difference. Slim is a young black man, an ex-con. Ridgeman is an old white man who has been a policeman for 30+ years.
Their similarities seemingly underscore the movie’s message about the core humanity of people, which supersedes race, background, occupation, etc. Whether black, white, criminal, or policeman, all are driven by the same needs and desires in the end.
As to why Slim is the one who makes it — I think the choice was made, thematically, because he was more innocent. Ridgeman was the one who broke his oath, showed little concern about the bank robbery deaths, and his police tactics are somewhat suspect (see the way he interrogates the Latino girl at the start). He, not Slim, is the one who gets anxious about the deal, mistakenly thinking that Slim would blackmail. Slim, in contrast, trusted Ridgeman and was probably gonna honor the 60-40 deal. I think that’s why the writers opted to have Slim make it.