Malachy was never meant to end up at the bottom of Lake Burion as part of the enchantment, though, honestly, I couldn't say I'm surprised. It's what I'd expect from a fairy like Carabosse. She has no self-control. The only reason King Geoffrey isn't at the bottom of the lake, too, is because Carabosse enjoys seeing him squirm. She meant to humiliate him as part of his refusal to accept her offer of a magical kingdom. When Carabosse found out from the midwife that Eglantine was carrying the eighth child, and, according to prophecy the final one the Queen would deliver, was a boy, her fairy-ship naturally couldn't leave well enough alone. Poof Off to the bottom of the lake the baby boy goes