"The arc of the moral universe is long, but it bends toward justice"---Dr. Martin Luther King, Jr.
What the rap lyrics, the strong arm robbery and sauntering down the middle of the street do suggest is that Brown could be aggressive and was an alpha male.
My theory is that when the alpha male officer gave a curt order, then cut back close to him and grabbed him after he was very slow to comply with a lawful order, the pumped up Brown pushed back and struck the officer (just bitch slapped away a clerk, now I bitch slapped away an officer).
While being struck, the officer reached for his weapon. Brown continued to saunter away ("punk" was dealt with). The officer then ordered Brown to stop, and another series of posturing and threats occured which resulted in the enraged officer firing at Brown based on a pretense (he did threaten to come back at me).