We know Lapis took the whole world's ocean to try and get to Homeworld. And that action was justified as she was only using it as a means to an end. She didn't intend to hurt Earth for very long. But the glaring issue many SU fans have with Lapis, or that many critics of Lapis has, is that Lapis tried to drown Steven and Connie, two children.
However, many of these critics fail to realize that Lapis is a Homeworld gem, and gems don't have children. They're just made, as revealed to us by Peridot.
With this in mind, and with Lapis under the impression that Steven is a part of the Crystal Gems (which he technically is but doesn't share the same intentions as Garnet, Pearl, and Amethyst) it would mean that Lapis didn't understand the concept of children. To her, Steven and Connie were just more beings that she needed to take out before they imprisoned her. She didn't understand the moral ramifications or standards around children, that children are practically defenseless and innocent. Because gems don't have children, the idea is incomprehensible to them.
Anyway that is how I justify Lapis Lazuli's behavior. What do you think of Lapis?
"We all want progress, but if you're on the wrong road, progress means doing an about-turn and walking back to the right road; in that case, the man who turns back soonest is the most progressive." ~CS Lewis