The link is 404, I don't understand why aw3c2 was downvoted.

404s do not contribute positively to the conversation. Is it not the case that comments are supposed to earn upvotes if they contribute positively, and downvotes othewise (including downvotes for mere noise)?

(Didn't vote either way, myself.)

Telling a poster their link is a 404 can contribute positively by prompting them to find an alternate URL. Otherwise, how would they know?

It could save other readers the time of loading a link that isn't going to work? I thought it was contributing in that sense.

He was -3 when I commented originally.

How embarrassing, I fail at reading comprehension. I thought the question was "I don't understand why tomerico was downvoted", and that was what I attempted to answer. The source of my confusion was that aw3c2 had net positive votes, and tomerico had net negative.