Well, it was your mistake #1.

Your mistake #2: the letter does not criticize Zionism (it is not even mentioned in the letter), but rather one specific political party.
And your mistake #3: you show Einstein as the only author of the letter, while there are about two dozens signatures in it. I doubt that Einstein initiated it or participated in its composition.
There is only 1 mistake on my part, that was to whom it was sent. In this letter, Einstein basically called Begin a fascist and accused him of creating a fascist state. He also inferred Zionists were acting like Nazis.
And I don't appreciate you trying to argue Einstein had nothing to do with this letter or its contents. Your argument is bullshit. Your argument is just your opinion, not a fact.
Everyone who signed that letter, endorsed its contents.