I would not send the letter but that is me.
Examine your motives for wanting to send this. Is it to help him or is it to defend yourself? What do you think you will accomplish be sending it? You seem to acknowledge it won't help him or so why send it? Do you really need to defend your actions? Do you have some hope of "them" turning to your "side" in all this? What would that mean? Is it likely?
Writing stuff like this can be very theraputic when done with no expectation it will ever be seen aby another human being. It lets you explore your feelings and thoughts.