Who secretly killed Theon's men in A Clash of Kings?

Question

In chapter 56 of ACOK, several of Theon's men are murdered:

Outside his door, Reek waited with Urzen and Kromm. Theon fell in with them. These days, he took guards with him everywhere he went, even to the privy. Winterfell wanted him dead. The very night they had returned from Acorn Water, Gelmarr the Grim had tumbled down some steps and broken his back. The next day, Aggar turned up with his throat slit ear to ear. Gynir Rednose became so wary that he shunned wine, took to sleeping in byrnie, coif, and helm, and adopted the noisiest dog in the kennels to give him warning should anyone try to steal up on his sleeping place. All the same, one morning the castle woke to the sound of the little dog barking wildly. They found the pup racing around the well, and Rednose floating in it, drowned.”

Who killed these men?

Answer 1

This is all spoiler to someone who hasn't read that chapter yet.

Theon.

The reveal is clear right after Farlen is executed for the murders:

I had no choice, he wanted to scream at the corpse. The ironborn can't keep secrets, they had to die, and someone had to take the blame for it.

There's a secret that needs to be kept.

He is burying the secret of who the flayed heads really belong to by killing off everyone that was involved. Reek is the only one that Theon trusts to keep his mouth shut, but he's not Ironborn.

Answer 2

Theon orders, but it's Reek/Ramsey that actually kills them, as Theon thinks later:

Theon had not heard him approach, nor smelled him either. He could not think of anyone he wanted to see less. It made him uneasy to see the man walking around breathing, with what he knew. I should have had him killed after he did the others, he reflected, but the notion made him nervous. Unlikely as it seemed, Reek could read and write, and he was possessed of enough base cunning to have hidden an account of what they’d done.