the first approximation is whether the set of 900 comments downvoted by user X would correlate more with "what other people downvoted" or with "who wrote those comments". That is, how much the user has high standards vs how much is a personal grudge.

That doesn't look like a good approach to me. Correlating with "what other people downvoted" doesn't mean "high standards" to me, it means "follows the hivemind".

Imagine a forum which is populated by representatives of two tribes, Blue and Green, and moreover 90% of the forum participants are Green and only 10% are Blue. Let's take Alice who's Blue -- her votes will not be positively correlated with other people's votes for obvious reasons. You're thinking about a normative situation where people should vote based on ill-defined "quality" of the post, but from a descriptive point of view people vote affectively, even on LW.

I think what you want is fairly easy to define without correlations. You are looking for a voting pattern that:

• Stems from a single account (or a small number of them)
• Is targeted at a single account (or a small number of them)
• Has a large number of negative votes in a short period of time
• Targets old posts, often in a particular sequence that matches the way software displays comments