Consider this before you jump. Consider Edwardian England. Rupert was moody. Perhaps not dark, but moody. Rupert was in love with love. I firmly believe that Rupert was infatuated with Michael. Were Rupert and Michael lovers? I think in Rupert’s mind yes. I don’t feel Michael wanted to make a commitment like Rupert pushed for. Loving and being in love are not the same. Casual sex and lovers are not the same.
On that day, it is possible that Rupert was leaning towards a lovers forever bonded in death idea. Poetic and even romantic in his mind. It is possible that he held Michael down under the water and then took his own life. Thus explaining why the two bodies were clutched together. Explaining many things.
There is no proof. Suicide is a possibility, but it doesn’t fit Michael’s pattern. Accident is a possibility, but again it doesn’t ring true. Too many questions left unasked. I think it was easier then to accept one of these theories than to consider the dishonour of Rupert taking both of their lives for love.