"Google just launched an Evernote killer" is a particularly inflammatory title for Hacker News. Almost all "killers" end up being competitors and more often than not "killers" never beat the product they're trying to kill in terms of userbase and revenues.
"Evernote competitor" seems much more appropriate unless there is clear evidence that Evernote is going to die as a result of Google Keep's release.
"Evernote competitor" seems much more appropriate unless there is clear evidence that Evernote is going to die as a result of Google Keep's release.