I heard Yoda killed a Sith but I know Yoda is not about killing. I've researched it, but can't find any actual proof. Has Yoda ever killed a Sith Lord or Dark Jedi Master and, if so, who?
Yoda never killed a Sith Lord, either in canon or in Legends. This is mainly due to the fact that there are so few Sith Lords due to the Rule of Two. The following is the complete canon list of known Sith Lords alive during Yoda's lifetime:
- Darth Plagueis (murdered by his apprentice, Darth Sidious)
- Darth Maul (thought to be killed by Obi-Wan Kenobi, ultimate fate unknown)
- Darth Tyranus (murdered by Anakin Skywalker)
- Darth Sidious (killed by Anakin Skywalker)
- Darth Vader (fatally wounded by Darth Sidious' Force Lightning)
There are of course Sith Lords who came before these and were alive during Yoda's long (900 year) life, but Yoda did not even know of their existence because the Jedi thought the Sith were extinct until the reign of Darth Plagueis. Yoda did try to kill Tyranus (Count Dooku) and Sidious (Palpatine), but failed. (Oh, the irony.)
An archived character profile of Yoda on starwars.com mentions that he killed a Bpfasshi Dark Jedi on Dagobah. Wookieepedia's article on the Bpfasshi confirms this. However, this incident is in Legends and therefore non-canon. I cannot find any other example of Yoda killing a Dark Jedi, either in canon or in Legends.
In summary: Yoda never killed a Sith Lord either in canon or Legends, he never killed a Dark Jedi in canon, and he only killed one Dark Jedi in Legends.
That said, it's not true that "Yoda is not about killing". He has killed other people, including clones:
In the Thrawn Trilogy, it's mentioned that Yoda killed a Bpfasshi Dark Jedi on Dagobah. This death created the dark side cave.
Somehow, Mon Mothma learned the story, and told Han and Leia. The Falcon went to see the Bpfasshi, to bring them into the New Republic, and were attacked by the Noghri.