A quizotic character is someone who pursues unrealistic and impractical ideals. For instance if your character is trying to make it so that people will no longer feel pain, or hate anyone. Both of these ideals are unrealistic, and they are impractical in the fact that if no one felt pain they wouldn’t know that they are alive and if no one hated anyone or anyone than there would be no need for people who love.
A quizotic character can be dangerous in some situations. Take Abe no Seimei from the anime of Twin Star Exorcists. His goal is to get rid of all the Kegare which are monstrous beings who reside in an alternate plane of existence. By doing this he would need to destroy all the bad things about humans and by doing this he would also be destroying all the good. This is why quizotic characters can make such great antagonists, because they believe that they are doing the right thing because of their ideals.
Quizotic characters are most often done well in dystopian and utopian fiction because often the societies in these stories are created by people with such strong yet completely unrealistic ideals. It may be a good idea to make a quizotic character change by the end of the story and come to the realization of the impracticality of their ideals. This could be a very interesting form of character development.