...apparently...or at least...Evidentally...
That's the problem: we are never exactly 
told.  (No disrespect intended, IluvBarnabas!)
Our esteemed MB said:
It's most probably true that there's no one scene where someone lays out the entire Dameon Edwards story, BUT everything about how/why his death occurred is indeed explained. The writer's provided all the details. And personally when it comes to a lot of the shows that I watch I'm often happier not to have the writers spoon feed me and/or tie things up with a nice neat bow because it means the writers trust the intelligence of their audience.  
 I disagreed with him on the desirability of spoon feeding, but kept my mouth shut on the extent to which the how and why of his death were explained.  I figured maybe I had forgotten some explanatory scene.  It's nice to know that, even if I did, I'm in good company.
[spoiler]I don't remember Dameon saying he was going to tell Quentin about Angelique.  I don't remember him saying anything at all.  I think at that point Angelique just wanted him out of hair.  As for the how of his death, I was never 100% convinced that Angelique had anything to do with it 
before the fact.  Did I miss something on that?  And as for the why, well, I don't know.  IluvBarnabas's suggestion that he was bothering Angelique to the point of murder seems unlikely, even for Angelique.  Surely a lovely lady like that knew ways of getting rid of an unwanted suitor without resorting to homicide.  Maybe he was blackmailing her?  (In that case I would immediately become 100% convinced that Angelique did instigate the murder.)[/spoiler]
I'm not sure why I added the spoiler warning, since this is all so speculative, but I figure I'll follow the crowd.