You MUST land the killing blow if you want the key.

Killed them both. Killed Pate first, then met with Creighton in Brightstone Cove. He didn't like that I was wearing a piece of Pate's gear, so he fought me. Merc'd him for it.

This is the world of Dark Souls. Ain't nothing fair.

I helped Pate because he's voiced by Peter Serafenowitz and the result is the same, but I think he's by far the more villainous of the two. The way I see it, Creighton is a pretender, a man who wants to come off as some intimidating foe but is actually just a tool who can be easily manipulated; his armour is outright told to be a replica, and he hardly comes off as a particularly smart guy. Pate on the other hand is a manipulator, constantly throwing you into dangerous situations under the guise of being hapless or not knowing any better. He wears pillaged armour and is referred to as a 'smooth-talker' by Creighton. Most importantly, he is obviously the serial killer Cale referred to; you meet Cale nearby Pate where you learn that he thought he recognised the face of a serial killer he thought was called "Cray-something". That's funny, because Creighton's face is covered by his helmet, and he's halfway across the world locked in a cell, while Pate's face is on full display and he's sitting a few courtyards from Cale.

My pure speculation is that Creighton WANTED to be a famous serial killer, and tried to ally himself with Pate to take his mantle, but Pate simply used that as an opportunity to manipulate Creighton into taking the rap for all the horrible things he did. Essentially, Pate is actually Creighton, and Creighton is just some nobody who wanted to be significant. For that reason, them both saying the same thing when they trick you at the end makes sense; with Creighton dead, Pate can walk away to continue to do his terrible work. With Pate dead, Creighton is free to take on Pate's role.