Why didn't Saruman kill the Ents himself?

[Sarumian]

Answering the second question, I tend to think that some confusion comes from the difference between the book and the movie.

In the movie a little force of Ents march on the almost empty Izengard. Although such a version of events seems consistent with the rest of the story, it significantly differs from the book narrative. In the book the army of Farngorn was much larger and consisted mainly of Huorns, tree-ish creatures, capable of thinking, moving very quickly and talking to Ents and each other (but not to other races). This army under the command of Ents marched firstly to Hornburg, where they joined Eomer's troops and together destroyed Saruman's army. After that they moved to Izengard and had it taken by storm before Gandalf and K arrived. Therefore, Saruman faced not just few Ents, but a formation of several dozens (if not few hundreds) of creatures who were determined and more robust than trolls, showing no mercy to their enemies. Saruman could hardly hope to discourage them morally or subject them to sudden panic as they themselves were capable of producing irrational horror in others.

As Zigûr says, most of Saruman's power was "invested" into his army and machinery. As I can understand the economy of power in Tolkien's universe, with the loss of the army this power was also lost for good (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.