Why didn't Saruman kill the Ents himself?

[Inziladun]

Originally Posted by Sarumian

As Zigur says, most of Saruman's magic power was "invested" into his army and machinery. As I can understand the economy of power in Tolkien's universe, this power was lost for good with the loss of the army (as it happened to Melkor in the end of the First Age). At that stage Saruman was much weaker than Gandalf the Grey facing the Balrog.

Zigûr makes some good points indeed. To that I would add that, as fire seems to be the only really effective weapon against Ents, Saruman was ill-equipped to deal with them as far as personal power goes. His main strength was in his voice, with persuasiveness. Gandalf was the one who was identified with fire.
Saruman might have had a better shot trying the soft approach with Treebeard, trying to talk him out of destroying Isengard (as his convincing Treebeard to allow him to leave later on shows), but for that to have worked would probably have required getting Treebeard to be both quiet and alone, and neither of those conditions would seem to be easy.