Sebastian is angry that Bloom keeps trying to set him up to be caught by Rosalind (see 1x05). He gets her alone and/or ties her up & suppresses her magic so he can punish her. He tells her if she lets him do what he wants, he'll reveal more about Aster Dell & her background, basically getting her cooperation in any way he wants to fuck/punish her. He does give her answers every time he's satisfied, so Bloom lets him take things pretty far.