We need a way to bail on the execution if we decide to.
Make it as easy as feinting an attack, or holding the same button you used to perform the execution.
It would help if we killed our opponent, and we're suddenly surrounded by their teammates.
We could stop the execution, and defend ourselves, or flee... Yes, sometimes you need to run, and live to fight another day.
The best you can do right now is drop target, which instantly pulls your weapon back from your opponent's body. What would be better though, is if switching targets had the same effect. I don't see any good reason why that shouldn't be put in place. You can accomplish the same thing right now, but not as smoothly as the alternative I just mentioned.