Is manslaughter a potential legal consequence of treating a patient without obtaining a valid consent?
i.e. A surgeon does not inform all the risks involved in the surgery to a patient, causing a death.
Because it is an unlawful killing and does not involve malice aforethought, I think it is manslaughter. Am I correct? Or is it just negligence?
I don't think that simply failing to make a sufficient explanation of the risks would make a death manslaughter. Three would have to have been serious negligence in addition, rather beyond the level needed to find malpractice, as I understand the matter.
Law.com says that:
Voluntary manslaughter includes killing in heat of passion or while committing a felony. Involuntary manslaughter occurs when a death is caused by a violation of a non-felony [sic], such as reckless driving.
The Wikipedia article on Manslaughter says that
Involuntary manslaughter is the homicide of a human being without intent of doing so, either expressed or implied. It is distinguished from voluntary manslaughter by the absence of intention. It is normally divided into two categories, constructive manslaughter and criminally negligent manslaughter, both of which involve criminal liability.
Constructive manslaughter is also referred to as "unlawful act" manslaughter. It is based on the doctrine of constructive malice, whereby the malicious intent inherent in the commission of a crime is considered to apply to the consequences of that crime. It occurs when someone kills, without intent, in the course of committing an unlawful act.
...
Criminally negligent manslaughter is variously referred to as criminally negligent homicide in the United States, and gross negligence manslaughter in England and Wales. In Scotland and some Commonwealth of Nations jurisdictions the offence of culpable homicide might apply.
It occurs where death results from serious negligence, or, in some jurisdictions, serious recklessness. A high degree of negligence is required to warrant criminal liability. ... An example is where a doctor fails to notice a patient's oxygen supply has disconnected and the patient dies (R v Adomako). Another example could be leaving a child locked in a car on a hot day
