Just ponder the order in which other things have to be added before it is reasonable to add executions to PVP.

First off, you would need a "downed state", which requires that there is a reason for said downed state, and logically such a thing would make sense of Apothecaries are added.
Once Apothecaries are added, you could have a downed state where you wait to be revived at the risk of being executed, unless you took terminal damage (i.e; headshots and such that would instantly kill you) making it impossible for you to be revived or executed for that matter.

Those are the prerequisites for an execution system in PVP, and I'm all for it. We just have to get the other parts first. And they've already talked about adding Apothecaries, so it's just a matter of time.