She tortured Jasper. Watch the end of Jailbreak. Watch Chille Tid, where she's in chains, gasping for breath, too fucked up to stand or form coherent sentences.
And she didn't fucking keep Steven safe. Jasper was weak, on her knees, she couldn't take on the CG, that's why she begged Lapis to fuse with her, because she couldn't win on her own. All Lapis had to do was step back and let the CG poof and bubble Jasper. Or pretend to fuse, but then once they were in the water she could unfuse and contain Jasper in a water bubble. Hell, we saw in Alone at Sea that she's fast enough to waterpunch Jasper into the horizon, through a goddamn boat, while she's mid-charge. They were next to the ocean, she could have waterpunched her into poofing no problem.
Instead, she chose to fuse, potentially giving Jasper access to their combined power as Malachite should she lose control of the fusion. It's a huge-ass gamble that puts Steven, the CG and the Earth at risk - and indeed, eventually Lapis did lose control and whaddayaknow Malachite almost obliterated the CG. It's not protecting, it's creating a huge-ass risk, all because she was selfish and wanted a prisoner to torture.
Sure she kept herself """"prisoner"""", but it was, again, a selfish thing - she didn't want to go to Homeworld, it was ruined, it had changed. She didn't want to stay on Earth with the CG who she despised. She chose to hide forever at the ocean floor, to stop being Lapis and become Malachite so she wouldn't need to deal with anything.
Jasper was just her teammate. They were both assigned to Peridot's mission by higher authorities. Jasper locked her up when it turned out she was a traitor who'd leaked info to the enemy and kept the disguise of the rebel general from them, what else would she have done?
Lapis is fucked up and has a lot of issues. She also treated Jasper in an atrocious way. You're not supposed to torture prisoners of war to work out your own traumas.