Freedom, you are right in that it is personal. However, my experience is they are highly violent and messy. There is a lot of noise, thrashing and crashing about, and many times do not result in a fatality. It ends up being just a bloody mess and attack and not a killing.
I have never seen or heard about any real world operational circumstance where someone sneaks up on a sentry from behind with a knife and silently kills them. I have also spoken with individuals from other countries where I thought it might be possible to have happened. They also had not heard of a documented case in their area.
The only silent killing I have seen done with a knife was in a video of prisoners who were passive being decapitated with a knife.
Usually, if the victim is able to struggle and fight at all in a knife attack – it is no longer a quiet affair.
Just my thoughts and I realize, I certainly have not seen everything. There may very well be times when knife killing could be quick and silent – I just haven’t run across them yet.