Looking for story based on quote; murder then suicide

[emptyheaven.livejournal.com]

I was watching Hunted (2x10) and listening to Dean's speech about "I may have to kill you, Sammy" and I suddenly had like a flash of something I remember him saying in another episode or a story I read somewhere. I can only remember bits of it but the jist is he's talking about having to kill Sam and he says something like "I'll stick around just long enough to see the light go out in your eyes and then that's it".
Basically it was him saying that once he had killed Sam he would kill himself.
 
Does anyone remember this? From either an episode or a story they've read?? I'm leaning more towards story thus why I posted here.
*tagged as best I could*