There's been similar bugs like this going around and I thought I may as well share an up to date version. (Apologies if it turns out to be a repost.)
In order for the glitch to work the person you wish to kill must be in a party/raid with you.
To do this glitch: You must play as a class with a dismissable pet E.G. warlock, dk, hunter and be able tank an 80 elite mob for a short period of time.
You must then head over to Utguarde pinnacle, once inside you need to sneak past the trash at the bottom of the stairs in the first room. The mob you will be tanking is the abomination at the back.
(If you don't kill the pack at the bottom of the ramp you must wait for the patrolling mob to move away from the abomination before you pull it)
Once you have pulled the abomination, tank him until your pet gets a debuff (disease) and then dismiss your pet with the debuff.
Now run out of the instance / kill the abomination. Here comes the fun part...
How to kill people with the debuff: To kill someone with the debuff they must first have about 3k health maximum, E.G. Bank alts, duelers, or just any odd lowbie.
Once you have found your victim invite them to join your group and then when they accept, summon your pet. The debuff from your pet will kill them!
Video link - YouTube - Kill anyone on your faction! (Video quality is awful but you can make out what im doing)
You can do it to duelers but dueling is dry for alliance unless you go dalaran sewers.
(If your horde durotar is perfect)
You can also try to be creative and "boost" some lowbies somewhere and unleash your pet on them.
Or you can duel someone, beat them and then summon your pet
PS: The debuff only lasts for about 30 seconds so be sure to dismiss your pet after you have killed someone, enjoy!