Sins of gedolim and Amalek

 Rav Tzadok has an interesting explanation of the mitzva of remembering Amalek

He notes the apparent strange condition that it applies after you no longer fear your enemies

He explains that Amalek is the Yetzer Harah and once you reach a very high level you think you are free. 

That is when you need to be extra careful and he cites the gemora where a number of gedolim -e,g., Rabbi Meir and Rabbi Akiva ridiculed those who sinned and as a result Satan would have gotten them to sin except for the special protection they had from G-d

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A similar drasha is found in the sefer Daas Torah by the mashgiach of Mir.

He notes the statement in the medrash that only a hair's width separates Gan Eden from Gehinnom. And thus even after reaching a high level you need to not be lulled in feeling you are out of reach of sinning as we see from the constant fear of the Avos