Finally I wrote how it all went down (I've been lazy to do it I must admit)
If something doesn't fit, or if there are still some things that I didn't mention and you would like to know, ask me and I'll try to think of something even though your guess is as good as mine because, and I repeat, the only objective of this riddle was who killed David, who was the accomplice, and the fact that David marked JM graffiti first and then alphabet graffiti just before he died.
Any questions?