Do you understand what mercy killing is lad? He didn't kill you out of mercy, he killed you so you STAY dead so you can't take revenge later. MK only applies if you are killing your friend or someone because you feel merciful, he did not feel merciful and wanted to make sure you stay dead so he finished you off.
Mercy Killing stands for : Killing your "OWN" Gang Members or Family Members Or Friends or a Player to Help them avoid a RP situation.
This is not Mercy killing, they are allowed to kill you and finish you to win the Gun store Robbery since you are the Opposite side.