Mr. Laser Beam wrote:
If Worf was allowed (albeit reprimanded) to kill Duras, then why would he not be allowed to deal with Kurn as he saw fit?
Was it because Worf killed Duras on board a Klingon ship, and this thing with Kurn was on DS9?
Worf was not allowed to kill Duras. Worf killed Duras, and then Picard chose to only discipline him by giving him a stern speech about being a Starfleet officer. Legally, though, the reason it wasn't prosecuted as murder was that it happened in klingon jurisdiciton.
Similarly if Worf had taken shore leave and killed Duras on a Klingon planet it also would not have been prosecutable by the Federation.