You can kill Kulle anytime by accessing his waypoint, but in order to get the chest, there has to be an official quest out to kill Zoltun Kulle. This quest isn't up all the time. You might need to open several games and control the Act2 bounties, to see if the Kulle bounty is active.
Remember there are 4 possible boss bounties in Act II: Belial, Maghda, Kulle & Vidian.
Only one of those will be active in an open game.