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.
Yet Worf killed Gowron on DS9
and he was never arrested for murder.
I think the reaosn why Worf got away with killing Gowron is because he not only removed his comm badge, the symbol that identifies him as a member of starfleet, but specifically declared that everything he says and does in that moment onward is as a member of the house of Martok and not a starfleet officer.
With Duras he just removed the comm badge then claimed later that what he did was legal according to Klingon law so he got a stern lecture form a disappointed Picard. But with Gowron he made sure to specify that he was acting as a Klingon citizen and not a member of starfleet.
Also Gowron had been screwing up the war against the dominion for petty political reasons. Sisko specifically told Worf to deal with the issue. And all of the top Klingon generals as well as their new leader were all happy with Worf for deposing Gowron so starfleet had plenty of good reasons to ignore Word killing Gowron.