It means You are the last brother...
Ommision of the verb in this way is called zero copula and is a common feature in African-American Vernacular English.
brother in this case is not meaning a literal brother, it is a slang term meaning essentially "(fellow) black man / African-American".
As for the idiom X is the last (person/thing) to Y... - I find this a bit hard to explain, but it essentially expresses the idea that X is not naturally suited to Y, or would ordinarily be expected to Y, and therefore it is surprising that they do.
For example, "Ghana is the last place I would expect to find a snowmobile manufacturer" (because Ghana is hot and not snowy) or "Chris Brown is the last person I would expect to see performing at a feminist festival" (because he's a domestic abuser).
As to why Mike is surprised about Marcus running down people, I am not sure without fuller context of seeing the movie.