Alexander and the Oracle

"Alexander's visit to the oracle at Siwa is one of the most controversial episodes of his life.  Ancient writers speculated endlessly on why he made the journey and what he learned there.  The details of the trip are conflicting, incomplete, and sometimes patently invented by those historians who wrote of it,  But in spite of the maddening contradictions in the sources, the simple fact remains that Alexander spent precious weeks in the middle of a war risking his life to travel across one of the most inhospitable deserts in the world to hear the words of a god."

Philip Freeman, Alexander the Great (2011).

The best reconstruction is that Alexander wanted to know whether the assasins who had killed his father, Philip, had already been caught and punished, or whether some might have escaped.  In other words, the visit to the oracle was CSI:Siwa.

The oracle told him that it wasn't possible for anyone to kill his real father, since that is Zeus.  But, he was further assured, the assassins of Philip had all been caught and he could put his mind to rest.

Of course, the person with the best incentive to have killed Philip  -- and the best incentive to want to put an end to conspiracy mongering on the subject -- was the one asking the question.