Anon Asks:

Why is slavery illegal in Westeros? We know that the northerners hate it. But why should it so hated as to demand execution?

Good question! 

One quick correction: both northerners and southerners hate slavery; it’s one of those few true universal taboos in Westerosi culture, like kinslaying or guest-right or oath-breaking.

I do think there are different cultural reasons for the shared belief, however. In the case of the South, I think it’s a combination of the Seven’s “all souls are children of the Seven” kind of ideology and the memory of the Valyrian Empire and the need to flee Andalos to avoid being enslaved. In the case of the North, I think it’s a kind of deep ancestral memory of the White Walkers and their drive to turn all that lives into their zombie slave army.