How easy is it to KO or kill your target with a completely unexpected sneak attack from behind?

[TSDTexan]

Richard "Dick" Marcinko in his autobiography wrote of ambushing a neighborhood bully from behind with a cinder block against the poor fellows skull. Striking many times, even after the KO.
The guy became a vegetable.

However, my older brother was laid out by a lead pipe back in the 80s from behind. He wasn't KO'd.... He recovered his feet and ran for dear life. Later had 13 stitches. He was pretty drunk at the time, and way more relaxed, which the Doctors credit with saving his life.

Knowing where to strike, and luck both have a lot to do with it. The element of surprise is potent, but not sure fire.

A scuba diver's bangstick loaded with a federal hydrashock sabot shell thrust into a guys sternum, or pelvis (femoral artery) would by more likely to permanently end a threat, then a knife to the back.

However, why would you really want to know OP?

 

[GiYu - Todd]

We typically try to teach/encourage students good situational awareness to avoid someone getting behind them for a sneak attack. Nothing is 100%, but just keeping your face out of your cellphone and looking around can make it tougher for most attackers to sneak up.
If someone is getting too close from behind, change directions/turn/maneuver so they're no longer behind you and you can now see what's happening to respond accordingly. This can be subtle in many circumstances and takes away the "attack from behind" advantage.
Not positive what OP had in mind. If he wants to know effectiveness of doing sneak attacks offensively, it's not something most schools teach since it's a bit cowardly. Would be a function of intensity and location of strike.