I'd concur wth the other guy here that reminded you how she wrote about the guns and the lifestyle she was involved with -- that alone is enough reason for you to cut your losses this time, maybe try to squeeze a little dough out of her for your time, and give her back those damn pictures and then blacklist her name as a model.
You are dealing with a mentally ill person straight up, yo.
You can go on and on about it being a business deal and so on like the other people here are suggesting but there is no such thing as a business deal with the mentally ill, and bro, she sounds FUBAR to me. That letter borders on psycho. This is the stuff stalkers are made of, trust me on this, I've been dealing with a stalker of this caliber for 11 fucking years, my man.
You WANT this chick out of your life and to forget you even exist as quickly as possible. Just try to get some dough out of her and regardless, give her back those pics and forget about it.