Rob245

Answer: Several reasons. Dobby was once owned by the Malfoy family until Harry freed him, which Bellatrix would consider a betrayal. She knows Harry is close to Dobby and killing the elf will hurt Harry. Dobby, as an elf, is magically powerful and a strong ally to Harry's cause. Having once been owned by the Malfoy family, Dobby has much inner knowledge about them and Voldemort. She was also making one last strike before Harry and the others escaped.

raywest

Thank you.

Rob245

Answer: She threw her knife at them as they disappeared. It's possible she hoped to kill Harry and not Dobby.