... I can’t dismiss the possibility that Michael hadn’t told Neri exactly why—but wanted him to overhear Fredo’s treason from his own lips so that Neri could be that much more motivated to kill Fredo at the appropriate time...
All the motivation Neri needed to do ANYTHING was that it was what Michael wanted hime to do. This is clearly indicated in the 'hugging' scene, where the look on Neri's face indicated he would not enjoy killing Fredo...but it was Michael's order and it was what had to be done.
