User blog comment:Plainblack/Blackbeard, the Enigma/@comment-9022101-20130902135815

Maybe he ate a fruit before the Yomi Yomi no mi that allowed him to steal other people's fruit abilities. Since we didn't actually see him with the fruit in its physical form, it is possible that Thatch ate it and Teach killed him to do the same thing he did to Whitebeard.