That's a bit unfair to show things like that. First, the tower needed to be destroyed. To be destroyed, they needed a big fusion. The only fusions who can fill that bill are Sardonyx and Sugilite, Opal being way too small. Second, she was under the cloud of incredible anger. Peoples act rashly when furious. It certainly doesn't compare with he kind of fucked up necro-rape Homeworld did with the gem experiments, that's for sure.

But it is clear Amethyst would have preferred to fuse with Garnet under better circumstances, not just as a back-up when I failed her. Anyways, it's the past now.