In the dialog that preceeds the Anakin vs Obi-Wan's fight
Anakin: You turned her against me!
Obi-Wan: You have done that yourself!
Anakin: You will not take her from me!
Obi-Wan: Your anger and your lust for power have already done that. You have allowed this Dark Lord to twist your mind until now... until now you have become the very thing you swore to destroy.
Is this to be taken literally? Is it included in the Jedi code an oath to destroy the Sith, or something like that? Or was it just a matter of speaking?