Horwath
Legend
While it is effective from the standpoint of scaring the players, I disagree that it's generally effective with respect to winning (and/or surviving).
You're wasting attacks to make certain that a character who can't attack you won't be able to attack you IF they are able to get back up. The thing is, if there are other characters who are actively attacking you (or your allies), then you are throwing away attacks that could move those characters towards not being able to attack you, over a maybe. Even if you are aware that they have a healer, it is often more effective to focus fire on the healer, to take them out of the equation. After all, with spells like revivify, even killing an unconscious character doesn't guarantee their removal from the combat. However, if the healer is out of the equation, it pretty much does (short of something like Wish).
Unless the NPCs are part of a larger organization, and are willing to sacrifice themselves to deal with that group's enemies, I don't see killing unconscious characters as the best means of survival. In the real world, soldiers don't typically go around executing unconscious people while enemy soldiers are shooting at them. It's illogical.
That said, this doesn't account for player psychology, only NPCs doing their best to survive and win. Obviously, play as you prefer.
this.
if you down a PC and then attack other one and beat him up, they will consider to grab their buddy and run of.
If you execute their buddy, they might just redouble the effort to cut your head off.
Now, we had in 3.5e variant that PCs used; when you move through a square a downed enemy(difficult terrain) you could as a free action step on their neck on the way and deal normal unarmed damage.
In 5e this could be an Extra failed death save.