Trying to understand character motivation with limited information is something I love thinking about. Lapis is one of the more recent ones. 

What we know:

• Pearl did not know the gem in the mirror was able to communicate instead of following orders.
  
• Garnet, Amethyst and Pearl did not know how powerful Lapis Lazuli actually was. During the fight with her, Lapis was able to overpower them all the the same time even though her gem was cracked.
  
• Pearl stated the mirror was found at the galaxy warp, the warp that connected Earth to the homeworld.
  
• Lapis Lazuli already knew who the crystal gems were already.
  
• Lapis Lazuli seemed to already know about the earth. “She never believed in it.” This makes me think she was placed inside the mirror while she was on earth for at least a little while.
  
• Rose Quartz’s power (the shield) completely neutralized Lapis’ power.
  
• Rose kept secrets from the gems.
• Greg’s speech about wars in general(link). Rose had to turn on her own kind.
  

I don’t think the current crystal gems could have put Lapis in the mirror. And based on their limited knowledge on Lapis, I don’t think they did it. However if ROSE did it to stop Lapis from reporting to homeworld to protect earth and kept it secret, it sort of would explain Lapis’ hatred of the crystal gems and earth. 

I know Rose also wanted to heal broken/corrupted gems, so I don’t think destroying Lapis would be the answer for her. She does care deeply for everyone. Then again its still a super cold move to do.

At the same time, I can completely understand motivation of both sides if that WAS the case. Turning traitor to homeworld, or protecting earth.

 That’s why Greg’s scene about wars really hit me.  It very possible the gems did things they would be terrified Steven would hate them for. War is an ugly thing and like Greg said before, there are no good wars. Both gems and people were lost because of it.

I really want to know wtf happened back then;;;