Caesar dead is more powerful than Caesar alive because Caesar alive is a complex human being, an ever-changing reality whom different characters respond to in different ways. Caesar dead, on the other hand, is a political symbol, a weapon, in the service of whoever manages to define him.
Is Caesar a benefactor to Rome or a tyrant in waiting? The whole justification for his assassination is that he is the latter, but Brutus makes the fatal mistake at Caesar's funeral of allowing Antony to sell Caesar to the Roman masses as the former, a symbol of Rome's greatness, and so to bring about Brutus' destruction. Thus, whatever one thinks of the reality of ghosts, there is justice in Brutus' lament that "O Julius Caesar, thou art mighty yet! / Thy spirit walks abroad, and turns our swords / In our own proper entrails." (V/3/94-96).
In the end, symbolic personalities are more powerful political actors than the human beings they derive from. Brutus recognizes this early in the play, when he wishes that "...we could come by Caesar's spirit / And not dismember Caesar" (II/1/164). When Caesar is killed, he enters the realm of the abstract sign, and thus becomes more potent in the service of his definers than he ever could have been when alive.
(See also M. W. MacCallum, Shakespeare's Roman Plays and their Background, "Julius Caesar" chapter 3, "The Titular Hero of the Play," available online but with too complex an URL to enter below.)