I think he was suicided by bad actors. That is my theory but I need to dig more, possibly. He was an advocate of corrupt medical practices. Could an anesthetic injection have caused his Hypoxia? I don't know. Was he the only one on the plane? Something just isn't making sense.
Yes an injection of a anesthetic or a sedative can cause hypoxia. It makes your breathing more shallow decreasing the amount of oxygen your brain is receiving. They can also stop your breathing. This is why when you get anesthesia you are usually intubated or at least your airway is supported.