There's one non-obvious link in the moderation logic here, which might help to explain a bit. We don't really moderate comments—we moderate subthreads [1]. In this case I was swayed by the fact that the subthread was 350 posts of especially-wretched flamewar hell.
I often put it this way: the value of an HN comment is the expected value of the subthread it forms the root of (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...). This is a weird idea at first—it was to me, at least—because how can I be responsible for what other people do? But when the genre is conversation, and it's relatively predictable how people will react, it's my (i.e. each of our) responsibility not to do things that will predictably generate poor conversation. Conversation is a dance [2]. We need to take the other into account, regardless of how right we are or feel we are. It's not really about being right; there are more important things.
For those of us who tend to think of ourselves as atoms, this is hard to learn, especially because internet posts feel like atomic operations. They're not, though—they're molecular, and the threads are the molecules. To be a good atom, you need to take into account what ions are floating nearby and thus what molecules are likely to form out of what you post. It's not that hard to know this—the dynamics are relatively predictable.
[1] Coincidentally I wrote about this yesterday, if anyone wants more: https://news.ycombinator.com/item?id=28932445