You are talking about "player assassination", right?:
• Player Assassination. By paying 10 gold, you are allowed to assassinate a Hacendado Card flipped over
to its “partner” side. This flips your victim’s card permanently back to its “Loyalist” side (representing his
son or heir). Note that Hacendados with Private Armies are immune to assassins
This would have to be done during your turn, right? It's not included as part of the Toppling Procedure section of the rules (although flipping hacendado cards to partner sides is).
Also, does anyone know why Player Assassination is not included as one of the actions explained in the Action Phase section of the rules, or on the player aid?
Player Assassination is no longer allowed as a normal action in the living (or Collectors Edition) rules.
Such assassination is now allowed only during a Topple attempt (and at a different cost than indicated above).