Avon only knew what he knew
Read a comment on YouTube about the downfall of Stringer and Avon. One of the best I've read:
Avon would have never called the hit in on D'Angelo. Stringer did. Just because Stringer wasn't concerned with his reputation didn't mean he wasn't cold, probably colder than Avon. Avon was a soldier who became a commander, he was hard but loyal. Stringer ran the business as a business, cruel and uncompromising.
When they worked together, with Stringer operating as the business manager and Avon working as the field commander, they were a perfect match. When they started working against each other, Stringer became the field commander and Avon got involved in the business. They stepped over each others' feet. Stringer ended up screwing up the fight with Marlo, Avon ended up ruining their business by fighting for corners. And, finally, they doomed each other.
Stringer needed Avon out of his way to fix the business and sold him out to the police. Avon got Stringer out of his way to protect his reputation and sold him out to the gangsters. Mostly, though, they forgot to include each other. Stringer wasn't just some weak yes-man, he was still hard as a rock. Avon wasn't just some dumb goomba, he was intelligent himself. They stopped trusting each other and listening to each other, then ended up ruining each other.
This is where The Wire became more than just a cool television show. This arc is on the level of a fable, a story of a landed lord and his trusted commander and friend who came up together to rule a kingdom, then eventually ended up being each others' downfall. It's important to note that Avon and Stringer didn't lose because of the police, or Marlo, or anyone else. They each lost because of the other.