With Obama being in London, I'd hop on a flight there. Buy some 2L bottles of water/juice, strap them to your body with gaffer tape, run at him shouting, 'I know you killed J.R. you ****, so I'm taking us all to the great Southfork Ranch in the sky!' I'm sure the secret service will put a round or 2 of bullets in you very quickly.
Plenty of ideas here http://en.wikipedia.org/wiki/List_of_unusual_deaths or you could be original and make it on the list yourself.
Leslie, you thought you could get away with it... http://www.bbc.co.uk/news/uk-england-lancashire-13554096