Why didn't they do it legitimately and not through illegal ways? What stopped them from contacting the Pakistani forces and working with them?

Obama praised Pakistan for its "close counter-terrorism co-operation". But officials said the US was the only country that knew in advance of the operation.

An official with Pakistan's Inter-Services Intelligence declined to comment other than to say that it was a "highly sensitive intelligence operation".
(source)

Because nothing says justice like killing someone that has denied any involvement in the crime he was "pegged" with, and whose guilt was established through numerous proven fake videos, and not even releasing the body for autopsy, or any kind of investigation/validation, because who was after anything but "justice".
http://www.salon.com/2015/05/11/everything_we_know_about_the_death_of_osama_bin_laden_is_wrong/