The ogre Korgrak can be recruited to be a hireling in your stronghold with the right dialog options. I was able to hire him, and then kill him and take his head in his cave. He did not die as far as the stronghold was concerned though, and he is in fact there in front of the keep when I traveled back.

Same here, I hired him then killed him out of curiosity, and Kograk still managed to make it to Caed Nua.