Jack Burden participated in Adam Stanton’s betrayal of his ideals. He convinced him to work with Willie Stark and unearthed the indiscretions, though noble, of his father. He also unwittingly handed his sister, Anne, over to Willie, thus corrupting everything Adam cared about. He appears to aim the gun at Jackie, but Sugarboy killed him before he could fire. By Adam’s standards, just like the woman from the north who brutalized her slaves in the south, the ‘friend of his youth’, deserved to die along with Willie Stark. Blot out the enemy, whip them, murder them, sell them down river. Anything but abide a knowing look in their eyes.
I believe he wanted it to be a double homicide.