Apparently some thing it may have been a suicide, other say it was murder, and some claim it was an accident.
Carradine was found in a closet, naked, with his hands tied behind his head, and a shoelace around his neck and genitals. This is apparently known as autoerotic asphyxiation, and it's supposed to enhance sexual euphoria, and officials say that he died because of it - by accident.
Source:
http://en.wikipedia.org/wiki/David_Carradine