The mob had Cyrus killed.
- Oh sure, Luther did the shooting, but what about that person on the phone? The mob wouldn't have let their power be threatened that easily, so they decided on a fall guy. The Rogues probably seemed perfect, with Luther's control over his people and his own cowardice. And even if the plot was found, the divided gangs wouldn't have been able to unite without a powerful leader, letting the mob get away scott free.
The Warriors, Streets of Fire and possibly Last Man Standing all take place in the same world, at different times.
it's more or less the "Walter Hill verse"
- The gangs didn't end up uniting, but they did outnumber the cops by a wide margin. After the Russians hit the city with a nerve gas attack, the government decides it's had enough. The seal off the city and make it a prison. Eventually, the Duke manages to fulfill Cyrus' plan and unites the city under gang rule.
His assignment was to infiltrate the gangs and take down Cyrus. He tipped off the cops to the meeting and his later phone conversation is with his handler. However, he was in so long that he became psychotic.
The police had Cyrus killed.
- Luther's contact over the phone is his police handler; he's likely working for them as part of a plea deal. Notice how quickly the cops arrive after Luther murders Cyrus, almost as though they were lurking just out of sight the entire time, waiting...