http://www.havenandhearth.com/forum/viewtopic.php?f=39&t=67070&p=846298&hilit=murder#p846298
The most recent update with regards to murder.
viewtopic.php?f=48&t=67039&p=845950&hilit=murder+knock#p845950Explanation of "Knock out protection".
Summary:
Knock out prot forces people to disengage when you get KOed. You must purchase this with a reasonably small numen points, and is consumable.
Knock out prot does not protect you from death from animals, and it only gives you partial protection from player killing. A player without murderous rage active will be unable to kill you if you're full HHP. The less HHP you have as a % of your total, the easier it is to kill you. A player with murderous rage active can however kill a full HHP person by dealing a large damage hit as their final blow (I forget if this particular part of the implementation works still, but there is some trick to it that someone else would have to provide), which implies they either massively outstat you or have a good guess on your total HP after armor damage. Anyone with the murder skill can always kill someone who has the murderous rage ability active.
TLDR:
You need to buy the murder skill to actually finish killing someone.
No knock out p = murder possible
Knock out p vs knock out p = Neither can die if full HHP
Knock out p vs murderous rage (removes knock out p) or no knock out p = Knock out p can die if hit properly, murderous rager/no knock out p can always die.
Murderous rage gives you the ability to possibly kill anyone, but comes at a cost. People without murderous rage active have better protection from death, but can't kill people with active knock out protection unless they have lost a notable amount of their HHP as a %.