Before we comment on the content of the letter itself, I will start out by saying that I am certain that the letter is a fraud. There is no way that a child, handicapped or not, wrote this letter. It's just not the way that a child writes or speaks. In that respect, it reminds me somewhat of the famous letter of Revital Avraham, which, like this letter, tries to emotionally use a person's death (although, in that case, it was a fictional person's death) to manipulate people emotionally.
The letter basically tries to make the case that Orthodox American Jews are bad Jews. An entire litany of sins (real and imagined) are laid out for us -- everything from talking during davening to eating sushi and pizza. Yes, of course, tznius is mentioned as well (has there ever been a tragedy in the past twenty years that wasn't chalked up to a supposed lack of tznius standards?), as well as laxity in kashrus, too much gashmius (materialism) and other items. He concludes by prophesying about terrible things coming in the next few months and that we must all repent our sins.
The kicker, of course, is that it isn't the killer who is responsible for Leiby Kletzky's death, but us. As "Moishela" puts it:
Q: Why is it a Kiddush Hashem if he was killed by a Yid?
A: Because it does not matter who killed him. It was the Goyishkeit in ourselves that killed him so that makes it a Kiddush Hashem. A true Yid would never kill a child as this man did, only if he is totally deranged. And even so, a real Yiddishah Neshomah could never be guilty of such cruelty; therefore he died Al Kiddush Hashem. The Goyishkeit in us is what killed him.
Personally, reading letters like this make me sick. It's one thing to try to advance your own agenda, but it's another thing to use a child's death to do so. If you want to make the case that Jews need to change their conduct in certain areas, then by all means, make the case for it. But don't tell me that Lieby Kletzky died because I ate a slice of pizza. There's only so much manipulative tripe I can take, and this letter went well beyond that. That's not to say that "Moishela" doesn't have some valid points. I think most of us can agree, for example, that talking and texting during davening is wrong and disrespectful. He may have some valid point in other parts of his letter as well. But when he wraps the whole thing up in an emotionally manipulative letter that blames everyone and everything except the actual killer (and lies about the authorship of the letter to boot), then I lose interest in the entire message he's trying to convey.