Let me run through a few things regarding this issue - First though, a disclaimer: I took no part in the so-called arbitrary rule applied to the UNMC, so this is coming from this post, as well as knowledge on what people have been doing in game (please note people, not UNMC as this isn't only about them)
In roleplay situations, the mercy killings turn out well sometimes, the person has good RP with the downed person(s), then executes them, which is a good thing, as (hopefully quality) RP has taken place. The main problem comes from people just shooting their teammate for the sake of shaving off the three minute respawn timer, which isn't so good.
The 3 minute timer is there for a reason - It adds some time before you can respawn, stopping people from immediately removing themselves from a situation for a small while, so that there is a chance for RP to take place.
In my opinion, a simple clause added to the RDM rule would suffice in fixing this situation - you need to re-initiate with someone after they have been downed, and RP with them before killing them. This prevents adding a whole new rule, prevents us blocking mercy-killings overall, yet removes the non-roleplay executions.
