I'll catch hell for this, no doubt... but from the moment I read one of the ricin letters I've been convinced that they was sent by a gay person. A white male, per Charles, but most likely a gay one.
First, look at what's not in the letter: no references to a higher power, nothing about divine judgment, no railing against sin, no "God Hates Fags," no choice quotes from Leviticus, Romans, Corinthians, etc.
The letters strike me as having been written by a very bitter man—by someone who came out, expected that gay life would a glorious cycle of song, and was shocked to discover that gay life—just like straight life—comes with no guarantees. In the years after coming out he learned that some people, gay and straight, can be assholes; that gay men were not, despite the hype, his "brothers." I wouldn't be surprised if this person had a meth problem and a string of failed relationships. He's someone who has probably, through the choices he's made, succeeded in making a complete hash of his life. But he doesn't want to take responsibility for his choices so he blames gay people in general, and gay life, and the bars, and pins his personal failures on the "community." (He's a blood brother to those guys who come out, spend ten methed-out years on their backs in bathhouses, and then decide that gay life is depressing and squalid and sinful before they "come out" as ex-gays.)
That would explain, I think, the letters' author targeting the bar crowd—he's mad at the folks who he believes are out there having a good time, drinking up, hooking up. Guys who are making a success at what he's failed at. It was probably at the bars where he met the guys who broke his heart, the guys who fucked him over, the guys introduced him to meth, etc. The letter was also written by someone who, if the threat is genuine (which I don't believe it is), can presumably move undetected through all 11 bars mentioned, someone who wouldn't look out of place at the Cuff, Purr, the Eagle, etc.
And the delight he takes in imagining the pain of his victims reminds me of the way cops describe the difference between someone murdering a stranger and someone murdering a lover: the degree of viciousness when the victim is a lover or an ex is usually much, much greater. Murder isn't the point; punishment for perceived wrongs and injustices is.
But maybe I'm biased: I get the odd death threat on top of a steady stream of angry emails. The most vicious hate mail I get—by a long shot, perhaps due to the narcissism of small differences, blah blah blah—is from other gay men. I get letters from Christian haters telling me they're going to pray for me; I get letters from gay haters—my, er, brothers—telling me that my boyfriend is a whore, that someone ought to rape my son and infect him with HIV, and that I should be killed. (This post will surely draw a few letters from my biggest gay fans.)
Kinks in my theory: the Wild Rose got the letter too, and gay men—bitter or otherwise—don't spend a lot of time at the Rose. So maybe the author of the letter is just a generic hater, not a gay hater; perhaps he's a not-particularly-religious resident of Capitol Hill who just hates homos. Or maybe he put the Rose on his list because he didn't want to be accused of being sexist.
UPDATE: Here's some pretty solid evidence that the author of the ricin letters is gay.