So why dont make that when you get the title "HAVE FUN AND BE EVIL" you become enemy of Horde and Alliance, that will create some sort of third faction of villains and would balance Horde and Alliance powers in WPvP. Also that will boost RP possibilities.
It would take some time to become friendly with your faction again, if you dont kill faction NPC or players. Also it will make bounty hunting more logic.
From the lore side I also dont see any problems. There is villains from both faction
![hiding_smth_turtle hiding_smth_turtle](./images/smilies/hiding_smth_turtle.png)
Summarizing: I just wanna have possibility to kill Alliance players while being an Elve as well as killing a Horde players.