Posts that get modded down to -5 should automatically have their IPs revealed.
Point one: circumstantial evidence
Just because a network transaction occurs between two IPs doesn't mean the hosts currently behind those IPs or their users were responsible for the offensive transaction. Your request (expose IPs of offensive ACs) is the meatspace equivalent of "hate speech sent through the USPS should have the return addresses posted publicly."
Letters and cards sent from buildings, and IP packets sent from hosts are circumstantial evidence. Condemning the owners or residents of a building, or the user of a host by its mail or network traffic is short-sighted and unjust.
Point two: free speech
Words alone are not harmful. Speaking out against a class of people is a bad idea, but it must remain protected speech for slippery slope reasons. We draw the line at threats only because they announce a destructive intention, and even then we respond to threats less strongly than we do to action. This is fair. Saying "X is bad" is not actually harmful to X. Saying "I'm going to hurt X" announces an intention to do harm which should be dealt with. Saying "It would be a good idea to hurt X" is a grey area which must be judged within a context to determine which side of the line it falls on.
When we fail to defend free speech our society becomes more brittle at best, and we give away power to cheaters at worst.
Point three: gaming the system
It is not particularly difficult to amass an army of Slashdot accounts with moderation points such that a single person could decide to apply -10 moderation to a post at will. Your proposal would grant such an army the ability to lookup any slashdot poster's IP. Do you know who is most likely to have the time and impetus to do this? The kind of person who trolls slashdot with racist nonsense.
You are asking to give control of the privacy of every slashdot poster to the kind of person who has no reservations about acting anti-socially.
TL;DR - End the witch hunts. Trollers gonna troll.