The Sefas Emes writes that the reason one can reach great spiritual heights on Purim even if he has not fully removed his impurities is because Pesach represents “ahavah”. On Pesach, Hashem’s love for Klal Yisroel increases and “ahavah distorts the line”. Therefore, Hashem increases our level of kedusha, even if we do not deserve it.
This is hinted to in the pasuk (Bamidbor 9:11): “On matzohs and maror, we eat it.” This can be understood to mean that even if a person is “like maror”, meaning that he still contains bitter elements of the yeitzer hara, he still can obtain the benefits of the Korban Pesach.