Jasper wasn't THAT hurt from the fight Garnet, what fucked her up was being thrown into the ship's core and that blowing up on her face. After that yeah, she had no way of getting out of it.

But yeah, Lapis probably wanted to punish herself in some way, or was too tired to think about the idea of going along with the cystral gems after what she pulled off before, so she figured "Might as well get us both down".

People give Pearl shit for martyring herself unnecessarily, and they have some merit behind it, but Lapis takes it to a whole other level.

Hangs back and just watches the situation escalate to an ugly level until she gets caught up in the collateral damage, goes out of her way to enact the most destructive answer possible, and blames the fallout on everyone but herself.

Seriously; Jasper was hurt, she had no weapons or bargaining chips to threaten anyone with, the Crystal Gems were 15 seconds of jogging away, Garnet had proven able to whoop Jasper in a straight fight, and they were on a coastline. Lapis could have just resisted or lashed out for half a moment and everyone could have walked away alright, she proves strong enough to overpower Jasper anyway. There was no reason for Lapis to pretend to go along with Jasper, no reason to do what she did beyond pure sadomasochism.

I really do think that: Lapis can't do anything unless it's in a way that hurts both herself and as many of the people around her that she doesn't care about as possible. it's the only coherent explanation for any of her plans of actions. She is the Patron Saint of externally damaging victim-complexes. No wonder Tumblr is so infatuated with her.