Well, this is not a comment on the particular situation (riding in the lane or to the side), but rather a thought that I think the OP was surprised that she had such a reaction to another cyclist and feels guilty. I identify with that, because it's happened to me. Mostly when the person (or group) is doing something blatantly impolite or illegal, but still, I feel badly that I am annoyed at a cyclist.
I am a timid descender, so I can see why some people would think it was safer to move right. I am pretty comfortable taking the lane anywhere, so if it's a curvy descent, you can be sure I will be out in the lane, because at least the drivers can see me. Since I probably will not be going over 30, the cars might get pissed, but I don't care. Most of the hills around here are not that long, so it's just a few seconds. This happens routinely on Strawberry Hill Rd., a major cut through street for both cars and cyclists, including big group rides. My street is off of this street and I have had to wait both going up and down the hill, if I am driving behind a cyclist. It's curvy and narrow and I won't put anyone's safety in danger.
The one time I did get really annoyed (yes, I wanted to *move*) was when a group stopped at the top of the hill at the stop sign and didn't signal, obviously didn't have good communication about which way the ride was going, and none of the riders were willing to put a foot down. They stood there, doing a track stand, looking like "where should I go?" I just waited, since it's a very busy and somewhat obstructed view intersection. The riders all started going in different directions! Good thing I waited longer than usual, because it took awhile for the ones who went the wrong way to turn around and continue on their way.
I just don't drive that way anymore on Saturday or Sunday mornings.