What are the guidelines for down-voting?
The tooltip when downvoting an answer reads:
This answer is not useful
That's the only guideline and it's open to individual interpretation.
Is it completely arbitrary to the user down voting?
Yes, in a sense. The only actual requirement to down vote is to have 125 rep.
Is right? Is this fair?
I think @maple_shaft's down vote is justified.
The question was extremely poor, any way you see it. You are not a Programmers noob, you are a 6k user and you should be well versed on the mechanics of the site. Your answer is of no benefit to anyone but the OP. And a "just a link" answer? Really?
Your answer could have been a comment, if you really thought that it would be helpful to point the op to that link.
Am I justified in going to his profile picking one of his answers at random and down voting for revenge?
That wouldn't be the nicest thing to do. And the probability of you getting caught is very high and the consequences severe - I've heard a rumour that Atwood delivers the spanking personally.... But if somehow you manage to not get caught, no one will ever know...
Except, of course, all of us who read the question were you advertise your intention to serially down vote...
Something that you may not know but it might be important: If a question has an answer, the OP can't delete it. So by providing an answer to a clearly poor question, you've robed him / her of the choice to later delete it.
That's something important to me, because my first question was a poor one, by any standard, but it got a few answers (and a few upvotes - it wasn't that bad). Now that I'm a little bit more familiar with the site I wish I could delete it, it's kind of pure noise.
It's a bit of a judgement call, but the question in question (pun intended) was clearly a very poor one.