HN's mandate is the gratification of intellectual curiosity. Intellectual curiosity is playful, wants to learn, wants variety. It is more fond of trinkets than of causes. Its relationship to More Important Things is itself curious: it inevitably touches on larger topics, yet if we were to swap priorities and put More Important Things first and curiosity second, it would kill the site. It would turn it into a fundamentally different thing, and you can't do that to something alive without killing it.
So the question is not "are there more important things in the world", it's "does a site dedicated to intellectual curiosity have a right to exist, even though there are more important things in the world". I think it does, because there is room for many kinds of website. This is just the kind of website that HN is.
If you accept that, you need to understand that such a site, simply by being interesting, comes under constant pressure to be something else. From a moderation viewpoint it's our job to protect it for what it is and resist the pressure to turn it into something else, even when we know the something elses are more important. Often we personally agree with the very things we moderate. That sounds weird if not impossible, but sheer quantity has a way of changing a person and you get pretty good at it after a while.
When you say "some of the ideas he thinks are bad are promoted and grow on HN" I don't know what ideas you're talking about. But if you mean ideological ideas, yes, they certainly crop up here, but I don't believe they're growing here. I think they're floating around everywhere. They also damage intellectual curiosity and so there's an easy argument that they don't belong on HN. Convincing everyone else of that of course is not so easy.