Maul was a Dark Lord of the Sith. You only become a Darth when you are one of the most powerful Sith in the galaxy. Granted there where only two known Sith at the time, Palp and Maul. Maul in the end was nothing but a tool. Palp was playing the long game. He did not believe that Obi Wan would have killed him but Maul was nothing but a way to test the Jedi's test and to make the Jedi doubt them selves. There was MASSIVE debate on if the Sith had returned. Many did not believe the Sith where coming back and this one rogue sith was just a random event. Yes there where Masters like Yoda and Windu that believed there where more but they didn't take a active role in researching it. As for the honor thing, let me quote the creator of the Rule of Two on the topic of honor "Honor is a fool's prize, honor is no use to the dead". In the end when maul was falling down the vat on Naboo that honor sure did do him a lot of good.
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