Snopes is reliable though (https://en.wikipedia.org/wiki/Snopes.com)
The other sites that are reporting it include:
yournewswire.com › Conspiracies
www.oversixty.com.au
www.disclose.tv
and the Daily Star, which ends its story with the line But the website behind the article...
Don't believe everything you read. What's the source of the Facebook story? Has it been reported anywhere else?
http://www.snopes.com/retired-mi5-agent-confesses-on-deathbed-i-killed-princess-diana/