Yes, we’re well-aware that Count Dooku is technically a Sith. However, his journey to becoming a Sith is disheartening, resulting in the fact that his death is both sudden and upsetting.
Count Dooku was originally the Jedi Master of Qui-Gon Jinn, having given up his royal familial life to become a Jedi under the tutelage of Yoda. Palpatine had made many efforts to lead Dooku to the Dark Side, but he failed each time. However, when Qui-Gon was killed by Darth Maul and Dooku was heartbroken over the loss of his friend, Palpatine swooped in as Darth Sidious.
Since everyone believed Darth Maul had also died in the battle with Qui-Gon, Dooku and Palpatine bonded over the loss of their former Padawans and Dooku agreed to become a Sith. If that was not heartbreaking enough, Palpatine later tosses Dooku aside for a younger model and convinces Anakin to behead an unarmed Dooku at the start of Episode III. Betrayed by a Sith master he tried to resist for years, still wounded by Qui-Gon’s death and too interesting of a character to be given only a brief appearance in the franchise, Dooku’s death was all-around upsetting and more than earned its place on our list.