Is it solely a matter of duration or not? Why is a pillow deadlier than a simple MMA choke (BJJ, whatever)? How does it work?
The two basic mechanisms when choking are blood and air. A skillful blood choke will knock someone out within seconds, say 2-20. A pillow choke blocks air, which takes much longer ~2? minutes. Death basically occurs when the brain has been deprived of oxygen for minutes. With a pillow, the person's brain is already oxygen starved when they pass out. If you hold a blood choke for minutes after the person goes unconscious, the expectation is this will result in death. This is why you let go. The person is still breathing with a heartbeat and will wake up.
As a martial matter, you always want to cut off blood. This will cause the opponent to go unconscious much faster than if you cut off air, when they will fight back with vigor to restore airflow.
In MMA (any martial arts really), the method is the ONLY difference. In martial arts or competitions, you do not go in with intent of murder. Even in public if you’re trained, and are attacked, you would focus on a sleeper hold with the bend of your arm around their neck while using your other hand to the back of their head to hurry the process of making the opponent (or criminal) lose consciousness so you win (or make a get away and call authorities).
The sleeper hold cuts off blood to the brain and so the other man can not continue until blood is restored and can become oriented of his surroundings and able to recall what just happened. That’s a controlled attack on your opponent. You win or you get away.
When choking someone to kill them, normally you are on top of them on the ground face to face, and you’re using all the force of your body to your hands and especially thumbs, and you’re putting pressure on their jugular. This stops air flow to the brain and the person can’t get air either. Even if you intend to put them to sleep and not kill them, this is not the way, because most times the pressure applied will break bones in the neck and possibly bust their jugular, and as mentioned above, can cause the body to also have reactions that can kill them in the process of being choked out (heart attack, Etc.).
Long story short, learn to use a sleeper hold if you’re that interested. It’s an amazing technique and easy to apply, ESPECIALLY if some idiot is show boating and decides to push you. If you know what you’re doing, let them push you and do a couple quick maneuvers, apply the correct sleeper hold, and they’ll find themselves waking up wandering what the hell went wrong. And also deterring anyone from bothering you again.