Okay, my actual theory is that Pink Diamond ordered her own assassination in an attempt to stop the war for earth. She had no idea the diamonds would retaliate the way that they did. She was just scared and she lost control and she didn't want to let anyone down, but she let everyone down.
The Diamonds were mad at her for losing a colony.
The colony was mad at her for what had happened.
She was losing a war for the only place she'd ever felt at home in and she was tired of it.
She thought that the only way to make things right was to let herself be shattered, honorably. But she was naive, and had no idea that the Diamonds would launch a superweapon onto the earth.