This is the oldest entry a quick google returns, was he copying an earlier snowclone?
"Creative unveils potential iPod killer"
http://arstechnica.com/old/content/2002/10/1152.ars
To add further confusion, I wouldn't assume that a Register headline to be without snark, and similarly I've lately seen iPad-killer used as a sarcastic insult, with the implicit assumption that it will fail.
Outselling implies that you are simply different. Killing implies that you have redefined the market by your awesomeness.
Google finds examples of Digital Research's GEM window environment being hailed as the Mac killer. This was back in 1985.
Searching for "IBM killer" finds references to the Digital VAX 9000, a failed minicomputer model that tried to beat IBM mainframes at their game.