To prevent chaos (not anarchy!), see Bava Metzia 83b.
Also, the Medrash says they were planning on pinning the crime on Esther.
See also Kedushas Tzion issue 41 page 6 by Rabbi Goldberg, that Mordechai’s plan throughout his stay in the diaspora was to convince the king to rebuild the temple.
(Forget the wisecrack by Rabbi Yehonasan Eibeshutz…)