When a convict with many feloniesi s offer a deal such as to be an informant?

how can the police dept and distract attorney take their word on what the say. such as let's say they where involved in a serious crime and they end up fleeing. due to their arrest history does included sex charges, robbery , assault and this crime scene has incident of all the above. my question is how can the police dept. and district attorney take the word from this suspect #2. as far as he stating he wasn't as crime scene.. of course this suspect #2 that fled crime scene is going to say what he has to to proof he is innocent