But is it clear from the followup post? Or do you just not believe the followup post?
There is no evidence that he slowed down out of vindictiveness. The OP seemed to feel guilty about taking pleasure from the effect of slowing down.
Right, and if the OP had stopped the car, taken a baseball bat, and smashed the tailgaters windshield it would have been wrong too. You’re responding to a hypothetical, not what seems to have really happened. The evidence at hand shows that the tailgater stayed close with brights on for 20 minutes. There is little that would make a reasonable person think that speeding up to a safe speed would help, or that stopping would not be dangerous.
And three wrongs don’t make a right either.
The initial tailgating and honking, when the OP was driving at the maximum speed that seemed safe to him, was wrong. Slowing down because of impaired visibility was not wrong, and, since it gave an opportunity to pass, was possibly even right. Not stopping in a travel lane when a possibly irrational person is behind you is definitely not wrong. Not backing off to see if being reasonable helps the situation is wrong.
The tailgater seems to have thought that 50 mph was a safe and appropriate speed on that road - and it might have been for him. It was not for the OP, and for the OP to have sped up to a speed that would have satisfied the tailgater would have been dangerous. Pulling over was impossible, stopping seemed to be dangerous, going 30 didn’t help, and going slowly didn’t help. I think the OP was out of options. The best compromise would have been for him to have gone 30 mph again, and seen if it helped, and then slowed back down to 15 if the tailgater stayed on his tail. Not being there I don’t know if that could have been done safely. I’ve driven during a blizzard where I would have loved to have sped up and lost the SUV on my tail but felt that I could not do so safely.