Spoilers below:



Just to clear up some misconceptions, the "Thought Bomb" that was used by Lord Kaan did not kill all of the jedi on the entire planet of Ruusan. When it detonated, it only killed individuals that were force-sensitive within a certain radius, not the entire planet. This has been cleared up in Darth Bane: Path of Destruction

Also, Lord Kaan did not come up with the "Thought Bomb". This knowledge was learned by Darth Bane from a Sith Holocron that was made by Darth Revan. Darth Bane then gave this technique to Kaan as an instrument that deceived him into helping destroy the faulty Brotherhood of Darkness in order to rebuild the Sith Order under the Rule of Two.

Yes, he was a genius.