You were casting unfairly casting your political opponents not just as wrong, but as morally reprehensible.

Right - and the interesting thing is, I had no idea that I was doing it, and in fact was trying to do the opposite. I did my best to take extreme viewpoints like "eating meat is like committing genocide" and "everyone should be converted or they'll go to hell" and attempted to portray them as psychologically no different from any other belief. But although I think I did okay with that, an uncharitable and exaggerated strawman still managed to slip in earlier on.

For the most part, I think it's just about the general ingroup-outgroup tendency in humans, and the desire to look down on any outgroups. But as for that bias slipping into my writing, even when I was explicitly trying to avoid it - that seems to have more to do with the way that most of our thought and behavior is built on subconscious systems, with conscious thought only playing a small role. Or to use Jonathan Haidt's analogy, the conscious mind is the rider of an elephant:

'm holding the reins in my hands, and by pulling one way or the other I can tell the elephant to turn, to stop, or to go. I can direct things, but only when the elephant doesn't have desires of his own. When the elephant really wants to do something, I'm no match for him.

...The controlled system [can be] seen as an advisor. It's a rider placed on the elephant's back to help the elephant make better choices. The rider can see farther into the future, and the rider can learn valuable information by talking to other riders or by reading maps, but the rider cannot order the elephant around against its will...

...The elephant, in contrast, is everything else. The elephant includes gut feelings, visceral reactions, emotions, and intuitions that comprise much of the automatic system. The elephant and the rider each have their own intelligence, and when they work together well they enable the unique brilliance of human beings. But they don't always work together well.

That elephant is very eager to pick up on all sorts of connotations and biases from its social environment, and if we spend a lot of time in an environment where a specific group (conservatives, say) frequently gets bashed, then we'll start to imitate that behavior ourselves - automatically and almost as a reflex, and sometimes even when we think that we're doing the exact opposite.

It is a pity that this kind of a bias hasn't really been discussed much on LW. Probably because the original sequences drew most heavily upon cognitive psychology and math, whereas this kind of bias has been mostly explored in social psychology and the humanities.