I wager you're talking about Tragic Prelude, the mural painted by John Steuart Curry.
To answer your question: Curry probably had Brown stand between the two sides to emphasize the increasing divide between the North and the South of the United States during this period, a divide that has only grown due to Brown's actions at Harpers Ferry.
Curry also probably represented Brown as such a large figure to highlight his importance in the leadup to the Civil War. He is also represented as a fanatic who would kill and be killed for his beliefs.
