Why did the robbers accuse Steve of the murder?

They accuse him of involvement in the murder in order to save their own skins. One of the main themes of Monster is the way in which the criminal justice system is less about justice than it is about making deals. And throughout his trial, it looks for all the world as if Steve is going to end up on the wrong side of one of those deals.
For instance, Sal Zinzi, already serving time at the notorious Rikers Island prison in New York, appears to have cut a deal with the Attorney General's office to gain early release in return for testifying against Steve. As a serial criminal, he's not exactly the most credible of witnesses, and Ms. Petrocelli does her level best to undermine his testimony. Still, Zinzi's spell on the witness stand does add to the drip feed of damning accusations made against Steve for his involvement in the murder. Even the words of a convicted criminal desperate to save his own neck can potentially sway a jury.