Let's imagine something. What if everyone in WoW could kill eachother. Not just faction against faction, but everyone against everyone. What would happen then? WoW would resemble the real world even more, so my guess would be that we would need some sort of policeforce in that case. I am sure it would turn into mayhem where the strongest would rule (aka ruin the game) through brute force.
Eventhough this scenario never has been, and hopefully never will be, a part of the game, other scenarios similiar to this one occur occasionally. Every now and then a bunch of people, usually guilds, decide they're going to run things on a server and set out to make life tough for the other faction. A friend of mine was part of such a guild. They were one of the most succesful raid guilds on the server back in vanilla (and probably are still today) and had the manpower and skills to prevent other guilds from entering raid instances simply by standing there and killing everyone who came close.
A guild on the server I am currently playing on used this policy as well, and they were famous for killing anyone of the opposing faction as soon as they saw them. They too could have gank fests outside raid instances every now and then, to the great frustration of every raid guild trying to have a pleasant raid night that evening. Most people turned to the GM's and asked for retribution, but without much result. Some repaid that guild by giving them some of their own medicine and ganking them whenever they tried to have a raid night. In fact, this guild became such a nuisance, something that could be called a civil guard came into effect.
It was never discussed officially, but somehow the entire horde faction (this guild was part of the alliance) decided that this guild needed to be taught a lesson. People had had enough of their bullying behavior. Everyone made it into a game to hunt people of this guild down and kill them, no matter level, class and spec. In the end it seems like this tactic succeeded. Having this guilds tag over your head made your life miserable and most importantly, the game unplayable. They decided to drop their "if it's red it's dead" policy and nowadays you hardly hear from them anymore. As for my friends guild, I'd guess they still like to boss around every now and then, but not as back in the glory days.
Another example of what can happen when people go out of control and start attacking everything they see is when Blizzard accidentally set loose a plague among the players. I've mentioned the "Corrupted Blood" incident before (but I have written so many posts even I have trouble finding them). When fighting the final boss of Zul'Gurub, players could contract a disease called Corrupted Blood. This disease would do a certain amount of damage each second, and also jump to nearby players. Players quickly noticed that they could leave the raid instance with this plague still on, and thus infect players outside of the raid, like say in a capital city. The only way to get rid of the disease was through getting a cleanse, which meant some classes were in a poopy situation if they contracted it. More about it can be read on the Wikipedia page, and it is really some interesting reading.
But it shows how people can act when they suddenly get the possibility to kill anyone around them. Most people actually had to try it out. Not odd, since a death in WoW isn't very final, but more of a nuisance. It is more to be compared with keeping something from someone for a period of time, rather than killing someone. But still, something you don't want to have to expect or protect yourself against from every other fellow running around.
Yet again, people found ways to counteract this new fad of infecting eachother. People who could remove diseases, like shamans, paladins and priests offered their services in capital cities for anyone who needed it. In the end however, Blizzard had to get in and change the way the disease worked. Imagine if the disease still worked in this way today and you had to be careful not to be infected whenever roaming in populated areas?
If that was still possible, or if as I started out, it was possible to kill anyone of any faction, people would probably respond in the ways they have. By setting up counteractive measures like civil guards against violence, and cleansing services against disease spreaders. It is funny to see that even in a game there will be those who try to wreak havoc and those who try to restore order.
Which side would you be on?