The question is 'does the intended victim pose me (or someone else) an immediate threat of violence, and is the force I am using proportionate to that threat?'
Note I said “ Use” in reference to Vampiric Touch, NOT, use on unconscious opponents,
as it seems, you are presuming.
I, substantially agree, personally, with the moral positions, you have advocated, in this thread.
If, however, I was fighting a Darth Sidious type villain, and knocked the Villian to zero hit points. Depending, on my role play choices for the character. I could see making the same choice as a Mace Windu, in performing a coup de grace.
Also, while the criteria I quoted above, from you, is admirable,
some will value tactical advantage as serving the greater good, more effectively than ‘fair fights’.
Military and Police units use ‘night raids’, precisely so the raiders catch the raid targets unaware, hopefully unconscious, and unable to defend themselves, effectively.
As a point of history, either a US Navy Corpman or Navy Seal executed a young prisoner of the battlefield of Afghanistan. Large swaths of the US population, including some notable leaders, had
no moral qualms, about this.
Which, is again to say, some may not see the issue as starkly, as you have detailed.
Good on them. They can be LE aligned like Bronson was in that movie. Or like the Punisher is.
Except, in their game, and view they are not LE.
Beside..
.....Outside, Once Upon a Time in the West, Charles Bronson characters strike me
as Chaotic Evil.

.
No regard for the law, never forswears vengeance, and is often sadistic.
The ‘good’ assassin, with extreme body counts is an American Trope, at this point.
There is no math, nor amortization loan calculations in The Accountant.