Is it okay to kill someone who's in a vegetative state?

For example, let's say a 35 year-old woman who had a good life is now in a vegetative state. She is kept alive by life-support machines, but she is unable to eat or drink, breathe on her own, speak, walk, or perform any basic functions. She has no awareness or consciousness and is unlikely to ever regain them. She had no living will and her family is unable or unwilling to make an end-of-life decision for her, but they continue to pay for the life-support machines out of a mix of love, guilt, and the hope that she'll somehow come back. Is it morally justifiable for a doctor to turn off the machines without the family's knowledge or permission, if that would be the humane and compassionate thing to do?