Quote Originally Posted by Malifice View Post
The head of a House (or even an important scion of a house) is far more valuable to you as a prisoner than as a corpse.

Traditionally in our own history, such high profile prisoners were held and then released as ransoms and similar. They werent just executed out of hand. Lords and Knights (as opposed to rank and file Soldiery) were kept as prisoners and ransomed off.

The only people in the show we have seen advocating for (and engaging in) the executions of POW's have been 'Evil' types (the Boltons, Sandor Clegane on behalf of the Lannisters, Joffery etc)
I really like your analysis, but in this you are prone to linear thinking.
What happened to house Tyrell? They were butchered to extinction, compliments of House Tarly & co. who betrayed their liege oaths.

Offering them the opportunity to bend the knee was far more merciful than anything they had a right to expect, and was very much in tune with "bad things happening to people who break the feudal rules" theme. Of course the show didn't do a vey good job of pointing this out, as it was busy analogizing Dany with her Dragons in that particular scene.