He doesn't need a deal, because if he testifies for the prosecution, and they secure a conviction of the other two on that basis, they will not be able to try to convict him of the same thing because to do so would mean that his testimony in the first trial was false and consequently the first trial result was in error. By using his testimony in the first trial they are implicitly accepting it as truthful, and they will not be able to turn around later and say, "no it was not and now we are going to convict you as well". If he testifies he will implicate the other two but say that he had nothing to do with it, he just saw stuff. So that would not help the prosecution to convict him in his trial, because to do that they would have to claim that he was lying in the first trial. For the prosecution, if it gets them a conviction of ZA they will have to accept DA's version of events, and that means they can get a conviction of the first two guys, or they can get a conviction of DA, but not both. You don't need a deal if the prosecution accepts that you are being truthful, and it is certainly in their interests to do that. But if he doesn't testify, then he will not be protected by that when it comes to his own trial. They can ask him to testify, and he can say yes or no, but if he says no then they will fry him with his own statements. He has to decide, does he want the other guys to go to jail, or does he want to go to jail. To overcome DA's testimony the defense would either have to show reasonable doubt that he is being truthful, or put their clients on the stand, which they will definitely not want to do. It is a catch 22 situation. The prosecution don't need to offer him anything, if he testifies the case against himself will go away simply because of how the system works.