Anyone under 1.0 is really awful at the game. People under .5 have very little clue how to play.
If they're not at least 1.5 they probably can't even put up a fight.
A high K/D is no guarantee that a player is good, but a low K/D is certainly a sign of a bad player.
3.0 K/D here, legit. There are only 2 players I've killed more than 10 times, so no stat padding or anything.