I don't know who wrote the letter but I do know it's almost paraody the killer went into court and had the prosecutor read the letter for him. You can't make this stuff up!
Guede even says it was all just a "thought in his head" and it's not up to him to decide who killed Meredith.
http://hellmannreport.wordpress.com/contents/reasons-for-the-decision/statements-of-rudy-guede-2/
But, apart from the consideration that to confirm a written letter to ones own attorneys is certainly not the same as responding to some precise questions on facts of the case, it should be noted that from the responses, given at the hearing of Rudy Guede, it is evident [risulta] that he has not implicated [indicato in] Amanda Knox and Raffaele Sollecito as the perpetrators of the crime having seen them personally in the act of committing it, but only because this is and has always been a thought of his. Quoting from the transcripts:
DEFENSE ATTY. DALLA VEDOVA And so, Mr. Guede, when you write the text that it was a horrible murder of the wonderful marvelous girl that was Meredith by Raffaele Sollecito and Amanda Knox what do you mean exactly? Had you ever said this?
WITNESS Well this, I never said it explicitly in this manner however I always thought it.
DEFENSE ATTY. DALLA VEDOVA So why did you write it?
WITNESS I wrote it because it was a thought that Ive always had [che è sempre stato dentro di me, lit. "that has always been inside me"].
DEFENSE ATTY. DALLA VEDOVA But then its not true.
WITNESS No it is absolutely true [verissimo].
DEFENSE ATTY. DALLA VEDOVA And can you elaborate better? What does that mean?
WITNESS Its absolutely true.
DIFENSE AVV. DALLA VEDOVA Do you confirm this fact [circostanza]? By [da parte]?
WITNESS Well, I with the
well, like I told you earlier, this is a thought that Ive always had in my head, its a thought that in any case in the end I decided to put in written form after hearing certain absurdities, in my opinion [secondo me] and I [get to] take on all the responsibilities [on] hearing a puppet manipulated by certain people, thats all. Therefore if I wrote those words its because they are [real], Ive always had them inside of me. Its not up to me to decide who it is that killed Meredith, I in the statement that I made in my trial I always said who was there on that cursed night in that house, therefore I dont think that Im saying anything new, I just put in writing my thoughts and I made them tangible [concreta], thats all. Therefore I dont see what other questions I should respond to
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So, also in substantive terms [sotto il profilo sostanziale], the indication of responsibility contained in the letter submitted does not represent the result of an account described in detail of a real story, witnessed [constatata] by the author [Guede] and reported in its particulars, but only the expression of a personal conviction, based on evidence unknown to us [basati su quali elementi non è dato sapere], there being an absence of statements made in this trial by Rudy Guede.