He was likely writing 'RBG', red black green, the colors of the Pan African flag, a Black Power expression. But it's "very hard to untangle his motive"…
Via NY Post:
The unhinged sniper who murdered five police officers scrawled letters in his own blood on a wall in his perch in a Dallas parking garage, a top police official said Sunday.
Dallas Police Chief David Brown said investigators are still trying to determine what former Army reservist Micah Johnson was trying to say when he scrawled “RB” on a wall.
“He… wrote some lettering in blood on the walls, which leads us to believe he was wounded on the way up the stairwell,” Brown told CNN.
“We haven’t determined what those initials mean.”