Theon.

Theon then frames and executes Farlen for the crimes.

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.

He is burying the secret of who the flayed heads really belong to by killing off everyone that was involved (except Reek).

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.

Theon.

Theon then frames and executes Farlen for the crimes.

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

He is burying the secret of who the flayed heads really belong to by killing off everyone that was involved (except Reek).

Theon.

Theon then frames and executes Farlen for the crimes.

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.

He is burying the secret of who the flayed heads really belong to by killing off everyone that was involved (except Reek).