Well, at that point, I'd argue that Maul was the "Sith pretender", since Dooku was occupying one of the two positions in the Order of the Sith Lords right now. Maul was an outcast trying to create his own self-proclaimed Sith Order.
Did Maul even know of Dooku's history at that point?
But he was a Jedi at one point, and we know how Maul feels about them.
Think it's worth noting that Dooku consciously chose to be a Sith, engage in mass genocide (including of the Jedi), xenophobia, exploitation, deception and perpetuation of overall suffering, after 70 years of life experience. If that makes him a Sith pretender, then Maul being groomed from infancy is even worse.
Short version: Sith are just arrogant.
__________________ “The galaxy must experience the pain of death and the rapture of rebirth as I have. I will bring chaos. It is time for war.”