Gandalf travels to Orthanc to take advice from Saruman, at which point Saruman reveals himself as a traitor. Why does he then simply imprison Gandalf on top of Orthanc instead of killing him? Clearly Saruman is still intelligent and you would think intelligent enough to know that Gandalf would not reveal the location of the Ring. Clearly Saruman is more powerful than Gandalf otherwise he would not be able to overpower him to put him on top of Orthanc and therefore it doesn't seem impossible that he would have the power and resources (Orc army for one) to kill Gandalf.
I realise it would make for a short and boring story with no Gandalf, but is there any reason in particular that Saruman does not simply kill Gandalf at that point??