Because of common sense and because Blizzard told you so already
Gameplay that involves Zone Disruption and malicious player killing outside of consensual PvP will not be tolerated on Hardcore realms.
By this, we mean deliberate action to hamper or significantly impede the ability of other players to enjoy the game, such as (but not limited to) repeatedly killing quest NPCs or kiting important quest mobs far away from their spawn location to prevent them from being killed.
Most importantly, this also includes taking actions to deliberately cause the death of another player, such as (but not limited to) kiting higher level or elite mobs onto or near other players with the intent that they take damage and die, or deliberately causing a wipe in a raid or dungeon group. This is not a PvP realm, and the normal PvP realm rules do not in any way apply to this realm. Outside of a normal Duel to the Death, consensual PvP flagging, or Wargames killing other players isn’t the core goal of Hardcore gameplay. If players are found to make it their goal to take part in this kind of disruptive style of play, we reserve the right to take immediate and decisive action against anyone found to be deliberately causing this disruption to other players on these realms, including permanent account closure.
*** This is not a PvP realm, and the normal PvP realm rules do not in any way apply to this realm. Outside of a normal Duel to the Death, consensual PvP flagging, or Wargames killing other players isn’t the core goal of Hardcore gameplay. If players are found to make it their goal to take part in this kind of disruptive style of play, we reserve the right to take immediate and decisive action against anyone found to be deliberately causing this disruption to other players on these realms, including permanent account closure. ***
Of course I’m assuming the scenario in question is a normal duel and not a duel to the death and that the place you MCed them off of was high enough to kill them.