Allen Goode's initial response to the knife kill question was that it was 1-hit kill, but thankfully in the comments section David Seymour clarified that answer and said:
The answer above about the knife attack is incomplete. The knife strike is a one-hit kill from behind, but from the front it only does half damage. Apologies for the omission,
Which is great news. We won't have to worry about people rushing us in a close-quarters gun-battle if they are going to have to take two strikes before they kill us.