Okay, so my last post actually received a response. It wasn't quite as much detail as I'd hoped for, but in truth it was more than I'd expected.
The response was, first, that my life is forfeit* for talking about God the way I do; and, second, that everything will be all right if I repent and turn to God.
Let me add a couple of caveats before I proceed. First, I do not guarantee that these answers were directed to me specifically. They seem to be, but... Anyway: second, as answers go, these are pretty oblique. They aren't a detailed answer to the questions I was asking, though I think they are an answer of sorts. It's probably also worth noting that he's addressing me in the plural: our lives are forfeit for the way we talk. I'm not sure whether this means that he's addressing my readers as well, or whether he thinks of atheists as a faceless horde sharing some communal essence ("We are Legion"). I have a very strong impression that he thinks of all unbelievers as atheists, and all atheists as being in active opposition to God**.
Which brings me to my next theory: maybe he isn't afraid of what atheists*** have to say. Maybe he's just offended. In this scenario, Dennis has a close, personal relationship with God. This is not so unusual; a lot of Christians have, or claim to have, such a relationship. So, naturally, when someone speaks insultingly, disrespectfully, or even indifferently about his friend, Dennis gets offended on God's behalf. That would explain why his rants seem to bounce back and forth between insults ("you headless idiot") and threats ("your lives are forfeit" - this may translate as, "you'd better not talk about my friend that way, or else!").
If true, this presents an obvious problem: God is quite capable or speaking up for Himself (c.f. the burning bush, Paul on the road to Damascus). Actually, this would be a problem even if Dennis were speaking up in defense of a human friend.
Also, this explanation doesn't make his posting/e-mailing behavior any more sane or reasonable. There are an awful lot of people who don't believe in God and are perfectly willing to say so; trying to respond to all of them has to be like trying to fix Hoover Dam with a roll of scotch tape. Also, if this is the case we're looking at someone who perceives God as, basically, a close personal friend (as opposed to, say, a still, quiet voice in one's heart), and who has a compulsive need to respond to anyone he perceives as insulting God.
Dennis, if you read this: is your goal to get people to stop speaking disrespectfully of God? Is that what you're trying to accomplish?
* Which is a nice bit of eighteenth century grammatical construction.
** As if that were even possible.
*** Etc. - I'm using his terminology, here.