It's better the way it is. And logically it wouldn't make sense if you could save a teammate from an execution unless you killed the enemy within a second of performing the execution.
As soon as that chainsaw is in, they are f***ed. No matter what happens.
Plus it would be very aggravating to be in cases where you shot someone down, ran up to their body before anyone could rez or steal the kill, then in the end not even get the killbecause some twit blasted you with a sawed off or something.