Update: Valve has reinstated Hatred's Greenlight page. Today, Destructive Creations posted their game "Hatred" to the Steam Greenlight service to see if the community would be interested in purchasing it. Hours later it was taken down for reasons unspecified by Valve.
Hatred was announced on October 16th with its first gameplay trailer posted online. The trailer for Hatred showed a man stuffing a number of weapons in a trench coat before going on a violent killing spree in the town. Violence in the world of video games is nothing new, even on the Steam store. From the early days of Wolfenstein 3D and Doom, to more modern games like Call of Duty and Hotline Miami, the mass murder of virtual human life is fairly commonplace.
So one is left to wonder why Valve has started banning games from sale now? Besides the above mentioned titles, Steam still sells plenty of more mature games through their service. Grand Theft Auto V is still listed as an upcoming title (despite Target and Kmart Australia removing it from sale.) You can still buy all three games in the Postal franchise on Steam (Full Disclosure: I received Postal 2 for free from the developers though #OpGameItForward.) Beyond just the violent side of adult content, Steam even offers Everlasting Summer free to download, despite being an M rated Eroge title.
Though it should also be stated that what Valve is doing is not illegal as The First Amendment only extends to laws made by the United States government. While it is debatable that what Valve is doing is wrong, there is nothing the law can do to stop them. Gamers who wish to play the game will have to purchase it from Good Old Games or Desura instead, assuming that those services don't refuse to sell the game as well. Though even selling Hatred through Paypal on the Destructive Creations website isn't necessarily a guaranteed option. Paypal has, in the past, banned and withheld money from users for selling adult products through their service.
And speaking of money, it says clear on the Greenlight submission page that "abuse of Steam Greenlight will forfeit your Submission fee and result in a ban from Steam Community Services." So it looks like the developers at Destructive Creations won't be getting their money back from Valve.
Update with a response from the team at Destructive Creations: