In "Legacy", the killing of Korah was as accidental as you can get. It wasn't a malicious attempt by Gabrielle to kill someone. She did so out of the fear of someone attacking Xena. So, she protected Xena. Not done out of anger. Self defense? Yes.
In "A Friend In Need", the burning of Haguchi and the killing of the 20,000 souls (or was it 40,000? :/) was (far fetched, I admit) was a malicious act. It was done out of anger, not self-defense (as the villagers were already retreating). And to boot, she was drunk. It was an inexcusable act done by Xena onto the innocent villagers who were just protecting their shrine.
Now we're onto the evil torment of the 40K souls who were punished to not go into a state grace after their "murders." It could only be solved by a (read it and say it) SELFLESS act of Xena.
The following exchange is from "One Against An Army" (an argument that one can hold up true to the Greater Good and not the spit that was in "Legacy")
Gabrielle: The first thing is the greater good. You taught me that. You taught me that there are things in life worth dying for. Things that hold a higher meaning than our own existence.
Gabrielle: A long time ago, I accepted the consequences of our life together. That it might one day come to this. It has. I'm not afraid.
Lord have mercy. Here we have Gabrielle saying a long time ago, she accept the consequences of their life together, that it might one day come to this. Gee...
Now, we're onto the whole vengeance/peace thing.
Quite honestly, the vengeance angle was weak and they should have left that plot out of the story. Might have gone over better. But since it is there in the episode, I'll address it.
There is no way that Gabrielle would deny Xena her chance at knowing peace. Gabrielle, above all, has wanted nothing more for friend to know Peace. Gabrielle would NEVER take that away from Xena. NEVER. She loves her too much to take that Peace away.
On the surface, Gabrielle knew that the reasoning was wrong. But deep down, she knew that Xena was going to achieve something: Peace. And like I said, Gabrielle wouldn't deny her friend that opportunity.
It is, as they say, simple as that. Again, the whole Legacy vs FIN track is wrong. It's weak at best. You're compraring the act of one accidental death to that of 40K committed in act of anger.