I don't understand why people assume Garling is going to betray the guy who gave him:
Immortality
regeneration
the highest known status in the world
the highest position possible for his son
probably saved his life generally
the most imaginable wealth and other resources he has to do pretty much whatever he wants
and he's pretty much seems to like the guy, as he's planning the "great cleansing", which is something Garling would probably like. If anything, he seems to be more into that stuff than the rest of the Gorosei, who are making these awkward faces while interacting with Harald