At the end of ADWD many people curse Theon for being an kinslayer.
The hooded man, the spearwives, crowfood Umber.
Reek/Theon is infamous throughout Westeros for being a turncloak, not a kinslayer.
Killing your foster brothers is considered a hideous act but not kinslaying.
If a hostage could rise to the status of a true member of his captor's family, killing him would be kingslaying too.
Yet nobody would accuse Ned Stark of anything if he chopped Theon's head off, that's why you keep hostages after all, to kill them if need to be.
Obvisiouly Theon didn't kill his father, uncle or sister but what about bastards sons?
Bevore Reek lost his manhood he was quite a womanizer who liked the taste of norther girls.
And the only time he had children killed was at the mill at the acorn water.
Children of the miller's wife he used to sleep with regulary.
And he is known not to be good at recognizing familiar faces, since he didn't recognize Asha back on Pyke.
My guess is he killed at least one of his own bastards sons.
And given his terribles nightmares and denials afterwards he must have came to the same conclusion afterwards.