It is not outlined very well the means of Mercy Killing an enemy. The idea behind it seems like it was ripped from DOOM, but lacks consistency in the execution of the kills. Also, what is considered an eligible enemy? And the health threshold is not outlined very well, as some enemies will be dropped to the threshold and can be Mercy'd, but others will not. From my testing, I have found if enemies drop below the threshold, they cannot be Mercy'd, so their health has to be at a specific percentage.