For what reason do you mostly down-rate people on iin?

Whenever someone unnecessarily lashes out at the OP.

I don't always down-vote but when I do you best believe that advice was absolutely terrible in every context ever.

I don't often, but if I do it's for idiocy or meanness that isn't quite reportable. Meanness in general isn't against the rules.

I don't downvote. I voice my disagreements on something I would downvote so that they know why I do not agree with them and/or they can perhaps explain to me why my reason for disagreeing isn't a good reason to disagree.

If someone's presenting an insulting comment under the guise of his or her actual opinion, it's worse than a troll comment obviously made to be funny. The only comments I've ever 'disliked' were comments intended to hurt or humiliate. Everything else is fair game; freedom of speech and all that.
If I don't agree with somebody, I might reply to them rather than give it a thumbs down.

I down vote someone if their comments are irrelevant to the question the OP asked.

I don't bother with it that often, since there's no point. However, I almost always thumb people down when they're whinging about getting thumbed down (because it makes me laugh)...and then I'll occasionally thumb down the sex offenders who can't comment in a post without making up some elaborate story about getting spanked by their cousin after getting caught sniffing their ass and then answering every question with dicksucking. I thumb these people down because they're too fucking stupid to bother disagreeing with.

if they try to be funny and fail at it or if they're an asshole

Because they smell like beans.

There should be an option for all the above.

Don't use it. I do check back on some of mine. No surprise I get a lot of down votes. Which is fine with me. Controversy breeds unity!

When they say some really stupid shit.

If someone is rude and being off topic that warrants a down vote from me. I will never down vote someone for having a different opinion to mine.

Unnecessarily judgemental
Racist or otherwise hateful
Factually inaccurate