Rosetta got bullied or something and got pushed into the swamp. As she is drowning, she harbored a lot of hate for her classmates and teacher. Then a necromancer comes along and brings her to life. Since Rosetta's lingering feelings are of hate and wanted to kill her hated ones, that's what she did (hinted by the fact that the Elfdragon incident whereby the Mommy Elfdragon still wanted to find her young one even after death due to her lingering feeling, then dies after fulfilling the feeling)