I believe that you can head shot with a melee attack, and that makes it a one hit kill. (This is speculation)
However, I have been killed by single melee attacks, unless I was somehow lagging and never saw/heard the first hit. I have also had people one shot me from behind with a melee hit again, lagging would be the only other explanation.
In summation, I have seen one hit melee kills, so it has to either be lag causing me to not see the first hit or blaster shots, or something is causing a melee hit to be instant kill, and headshots are the only thing I can think off.
Edit: As a side note, it MAY also be back attacks as Dupree3 mentions, but if so, it must be a DIRECT back attack. I have hit many people from what i thought was behind, and had it not be a single hit kill. I had to bash them again