Octavia has always wished to be free.
Bellamy knows this, and because she's Octavia, he also knows it will happen.
For a second child, things always turn out pretty well for her.
Every night, he regales her with tales of moonrises and sunsets and concerts and the biannual feasts, which is just everyone throwing their rations together and enjoying themselves, but used to be his favorite times of year.
He never tells her that moonrise is the most common time for a floating.
He never tells her that concerts and feasts frequently end in hungry children being quietly taken away for taking more than their share.
He never tells her that it's often him pressing the button and shocking the children to unconsciousness.
Because if he tells her this, she might lose her reason for living: she may one day be free.
Bellamy knows that she won't. He knows that she already sees the darkness of the Ark, in the way the guards speak to their mother, in the terror that comes into their mother's eyes when Octavia talks of leaving, in him when he comes home after doing horrible things all day.
He's begun to snap at them, pick fights. He knows he's acting just like the other guards do, just as terrible as they are.
Still, he wants to keep her safe, shielded.
What she doesn't know is that freedom is where the demons are.