One popular theory, is that the Somerton man committed suicide, possibly with some assistance from Jestyn, who may have acquired the poison for him (or possibly because of Jestyn after being rejected).
One piece of evidence which supports the suicide hypothesis is the “Tamán Shud” slip found in the man’s fob pocket (Tamam Shud meaning 'It has ended'). It may have been hidden in his fob pocket, but the Somerton Man knew it was there, and he knew it's meaning. Despite what TV may have us believe, only 25-30% of suicides are actually accompanied by any kind of a note (and who knows, maybe the code was his note).
The Rubaiyat contains poems focusing on living life to the fullest and not being sorry when it’s over, which is kind of fitting.
The labels were not only removed from his clothing, which a murderer could have done to prevent identification of the body, but they were removed from his suitcase and all of its contents. So unless the suitcase was a plant he must have done that himself, before leaving the case at the train station.
He had no significant bruises, injuries, or defensive wounds that would normally be present if he had been attacked. The pasty that made up his final meal contained no detectable poison. It seemed that, whatever the cause of death was, it was self-inflicted and not administered through force or secretly poisoning his food.
The that fact that no signs of a struggle, convulsions, or vomiting, found at the scene does support the suicide theory. He may have taken his poison at the water’s edge, thrown its carrier into the ocean. Or he may have taken the poison before even reaching the beach. Convulsions aren't always massive fits, they can be quite subtle. The movements of the arm described by J.Lyons would be consistent with convulsions.
If he had vomited elsewhere then there would be none at his final resting place. If there had been signs of a struggle, these could well have been obliterated by various people trampling all over the crime scene. By the time Mr Lyons, the strappers, Constable Moss, and any other nosey on lookers had all taken a gander, not much would be left.
People taking their own lives often chose public spots, this is so that their bodies will be found.
The position on the beach faced west, so he may have been watching the sun set over the ocean one last time.