His best friend had just betrayed his family, not just Avon. Notice throughout the whole show Avon is always connected to family. His only mistakes are mistakes linked to family, because no one else can get that close. Avon is already a multimillionaire before the cops know he exist because he's in the shadows. They know Stringer Bell exist though, because his goofy ass is hanging out in the court room for a bloodhound like McNulty to sniff out
Avon's rise and power is connected to his family. Remember when they visited his brain dead uncle; "...and how you ain't gonna never be slow, never be late?" Brianna Barksdale is apart of it too, because its a family business. We even see Avon's girlfriend and one of the places he lives with her. We see D'Angelo's girl and baby, and he even reference their grandfather being in the game. They are a criminal family.
I say all that because its an important distinction between Avon and Stringer. We never hear about Stringer's family, see him with anyone he loves, or know much of his history. Stringer is a man without a country, whose only real loyalty is to himself.
Avon and Stringer parting ways was only a matter of time, because despite their success as a team, they were very different, but that difference also made them a dynamic duo; the gangster and the con-man. In the end, they both had been beaten by better gangsters and better con-men.
