She can see Lapis trying to summon her wings and failing, and it makes this feel so much worse. It's not like Jasper has never taken something out on a defenseless victim -- it's not like she's never done that and enjoyed it. She enjoys a fair fight more, but she is also, frankly, a bully, and she isn't at all above kicking people who are down when she needs to feel better about herself.

This isn't like that, though. Whether she's attacking Lapis or grovelling at her feet, it's because she wants to be with her, or for her to lash out in a way that might feel the way their fusion did. If Lapis can't push back against her, turn all the water in the room on her -- what's the point?

But the Lady of the Lake has compelled her, and so she has no choice but to advance on Lapis and seize her while she's still trying to deal with not being able to use her wings. Without her powers Lapis is all but helpless, and Jasper only needs to use one hand to seize her and plunge her underneath the water.