OK, here we go again. I typed this up earlier today and then after "previewing" it, hit the back button, deleting the entire entry. You would think I would learn.

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After Anakin kills Count Dooku, Palpatine tells him:

"You did well, Anakin. He was too dangerous to be kept alive."

At that time the audience and Dooku know what the Jedi do not.

Palpatine is Lord Sideous.

Count Dooku was too dangerous to Palpatine to be taken alive from the Invisible Hand. I'm sure Anakin didn't read any deeper meaning into Palpatine's comment than "Even if a prisoner, Dooku will be a rallying point for the Separatists. He is a danger to the Republic."

Palpatine could not afford for Dooku to tell the Jedi that Palpatine was Sideous. Anakin wasn't ready to be brought over to the dark side. There were still some ideas to plant in his mind. His plans were not yet complete. Palpatine was nothing if not patient.

If Dooku had been taken as a prisoner of the Jedi, would the Jedi have avoided Order 66? Would Dooku have told them how the whole war was a setup? Did he even know about Order 66? Heck, Dooku might have known about the other 65 Orders, too. Certainly Palpatine thought Dooku was too dangerous.

Anakin may have been afraid that Dooku would end up escaping, but Dooku might have given the Jedi useful information under interrogation. He had already told Obi-Wan Kenobi that there was a Sith in charge of the Senate, but Obi-Wan didn't believe him. Part of the problem with interrogation is that you gain lies as well as truths from the subject. The trick is to figure out which is which.

No matter what the possibilities had Dooku survived, Anakin chose to kill him. Anakin's own vigilante justice fed the growing seed of darkness inside Anakin.