People use the downvote on HN for disagreement all the time. People don't like critiques or dissenting views, and some fraction of OPs will downvote a critical response. Some other fraction will downvote maliciously or selfishly.

I think the downvote should be for low-effort or low-contribution comments. I abhor "^this" or "lol", nevermind trolling or unfounded disinformation.