I think a friend bashed his head. If it was clear that he was dying, and in pain, it would have been the kind thing to do. If they were under pursuit, and Otzi was moaning in pain, it could also have been the necessary thing.
Such a friend might also have taken the time to drag him to a protected spot and lay him on top of his belongings in the hope that they wouldn’t be stolen, and he could come back to do a proper burial when the danger had passed. But would also have left his most valuable things with him, to ensure he’d have them in the next life.
Friend didn’t make it back.
Otzi could also have been part of a larger battle scene. Conquering heros walked through the area bashing the heads of the slowly dying. Otzi’s position on top of his valuables would have protected them from discovery if the scene was large and anonymous and the heros were rushing through in pursuit of the retreaters.
As for the arrow shaft, a crow could have taken it fifty years later. There’s no certainty that it was removed by a human near the time of his death. But if it was, either a friend would have removed it trying to help him, or a conquerer would have taken it for attaching to a new arrow head later on.
There are references in fairly modern fiction (one in Jane Austen somewhere I think) of a recreational hunter going to great lengths to retrieve a “favorite” arrow.
He was not especially on top of his belongings to hide them. They were in the open with him. His body was not placed in any sort of position that a friend might leave someone they care about. It was lying awkwardly on a boulder with the arm at a strange angle under his body. It does not seem consistent with “a friend left him with his possessions to return later for burial.”
That’s actually a popular misconception. Otzi’s corpse still has a willy. It wasn’t noticed at first because it had shrivelled up so much from 5,000 years of freezing. But he is missing one buttock, thanks to the jackhammer and ice pick that were used to dig him out of the ice before it was known how old he was. His excavation from the ice was literally a half-assed job.
