You betray your benefactor because you think it's all just nobles playing in their little world, not something of consequence. Turns out you helped a massive conspiracy get what it needs to kill another of its enemies
It seems to me that they are up to something criminal and to get away with it Second needs to be unaware exactly because he is the one in charge, there are countless examples of people getting away with crimes because they made sure to never have the orders come out of their mouth and made sure...