Honestly, if you've killed someone once, but they're still a problem, you might want to rethink the effectiveness of that strategy.
Palpatine: Anakin; you are pure evil. You destroyed the Jedi; corrupted the Senate; brought tyranny to the Galaxy.
Palpatine: I've killed you once; I can kill you again.
Darth Vader: But... Obi-Wan killed Anakin.
Palpatine: Oh no, I'm afraid he was still alive when I found his burnt body on the ruined lava fields of Naboo; that allowed me to extract the midi-chlorians to connect you to the Force.
Palpatine: I needed a lieutenant of unparallelled acumen; but alas, Jar Jar turned me down.