[X] Let Svetlana do it. After all, deep down, you know it to be true that you have to do ignoble things to get ahead in life, sometimes. What do the lives of a kinslayer and a greedy fool matter compared to your own safety and the hard work of someone you appreciate? Not even like it's the first time you've seen people die, either. You could do good things with all the riches they leave behind.
[x] Convince her to make them swear an oath to her. You agree that they can't leave here as is, but death is a little extreme. Curses and geasa are fine enough to insure they'll not be able to speak of this to any interested shikari without great repercussions. (Charisma+Presence)
[X] Convince her to spare them and convince them to never say a word of this. Get around the root of the problem and make it so they don't bother ratting you out of their own volition. You think the Guildsman would be easy enough, but the White Moth will be a tougher nut to crack; she's an enigma to you so far. (Charisma and Manipulation + Presence, Perception + Socialize).