Omitting many pages of confusing linguistic jibberish, what happened is that the distinction between the transitive and intransitive verbs was lost, the two verbs became a single verb, and northern England and southern England had different forms. By the sixteenth century, the northern form hung penetrated into general English and became the dominant form used--except in the case of execution by hanging. Presumably hanged continued to be used here because judges would use this older form when sentencing convicts (legal language, like religious language, is usually very formal, and more resistant to change than other varities of English).


http://www.randomhouse.com/wotd/index.pperl?date=19961107

It's both, for example in the past tense Ruth Ellis was hanged on July 13, 1955,
However if you were around at that time you would have said Ruth Ellis will be hung on the 13th.
It has to be either way as both are correct grammer

"You might as well be hung for a sheep as for a lamb"