Wrong region Killer Is Dead key

Contact the giveaway creator, and allow him/her to delete the giveaway.
It's the better solution, as that game has obnoxious region locks.

Request a new one form him, if he ddint state a region lock on his ga he must provide tha Game.
Or you allow him to delete the Ga

Contact the creator to see if they're willing to replace it with another key.
If they cannot, there are two options -- mark the GA not received after 7 days or allow the creator to delete the giveaway (the nicer option).

Comment in the giveaway to alert the creator to the situation and file a support ticket. You're probably out of luck on getting the game unless the creator decides to buy you a new copy, but if not, the giveaway can be cleared out at least. Honest mistake on their part, too bad.

So how do I contact the creator? I can't find where is the button to send a PM to him (WOW I feel like a total noob)

What Tzaar said! And let me know how it has ended if you don't mind :)

Ok, I left a comment on the GA and filed a support ticket. Waiting for response

Update here: I haven got no response for the creator. Therefore I thought it was the best to create a giveaway or trade the key, and the rest of the story is here https://www.steamgifts.com/discussion/4Yh9b/quick-take-this-killer-is-dead-key#S5vqLbs