Laß dich vom Bösen nicht glauben machen, du könntest vor ihm Geheimnisse haben.
Do not let Evil make you believe you can have secrets from it. [Kaiser/Wilkins]
Don't let Evil convince you you could keep any secrets from it. [Hofmann]
A cancelled aphorism, possibly slated for revision.
More good advice; William S. Burroughs used to say "nobody does more harm than people who feel bad about doing it." Why? Because they harm from a position of official justification, which is the same as saying they harm officially.
The misconception Kafka wants to clear up is that it's possible to do evil without being evil, or to do just a little evil, or to manage evil somehow; his point is that this idea is already fully evil. It's not evil you can keep secrets from, it's you, or rather, you have the power to deny or obfuscate or rename things about yourself or things you've done. Evil afflicts you by turning you into a false image, and that might be the falseness to which the trueness of the true way is opposed. The true way isn't something claimed and owned, it's a method of patient checking and attention.