I mentioed a while ago that I would post this, and here it is.

FOREWARNING: This was an extensive experiment, be prepared for a long post.

Overview: I've spent many hours farming reputation with various factions, and one day I said to myself "Why am I farming useless junk for idiots that I don't like, for near-pointless rewards? I should backstab them all." While I had originally planned to 'un-farm' my reputation with the factions I'd helped, I realised that this too would be time-consuming. So I just killed my way from neutral to hated on a different character (in most cases). Here is what I observed during my massacre of the various factions I dislike;

1) Booty Bay (Hunter)
Side note: I maxed out bloodsail rep several months before the achievement was out, and yeah, I'm extremely pissed about the number of scrubs who are only farming it now for the title. But that's off topic.

While killing goblins in booty bay, I observed the following:
Alliance: Most alliance players stood around wondering what I was doing, and watched for a while. This lasted (pre-patch) until I killed them with snake traps. A few of them tried talking to me, indicating a lack of game-experience, while others even tried to duel me, or flag and be hit by some of my multi-target spells. Eventually, they all left me alone, as they couldn't stop me, or because I killed them.

Horde: Horde players I encountered often cheered me on. The occasional few would just watch me for no reason, buff me, and/or heal me. Oddly enough, nobody asked me why I was doing it.

2) Timbermaw Hold (Rogue)

Nobody cared, even the people who passed me in the tunnel as I was doing it.

3) Cenarion Circle (Rogue)
Side note: Killing cenarion NPC's in silithus is unwise, as the guards are 70 elites. Moonglade is a much better choice. Killing moonglade NPC's will flag you. It will also broadcast that moonglade is under attack, on the local/world defense channel.

Alliance: Two druids appeared. One tried to heal the NPC's that I was killing, only to realise that he couldn't target them. After which, he got bored and left. The second druid, and undergeared resto druid, actually tried to attack me, apparently unaware that the guards would kill her after I vanished.

Horde: Two druids appeared. Both wandered around until they saw me killing NPC's, then left.

4) Ravenholdt (Hunter)
Side note: I somewhat liked this faction on my rogue, so I used my hunter here. (-3000) RH rep = (+100) Syndicate rep.
- Unlike bloodsail, there is, literally, no benefit to gaining syndicate rep, other than not being attacked by their mobs.

This part of the experiment isn't actually over, but I can predict the results. Due to the lack of access to the area, the fact that attacking it doesn't prompt a local/world defense ping, and the overall uselessness of the faction, I probably won't ever see anybody there.

5) Argent Dawn (Rogue)
Side note: I wasn't properly geared to kill NPC's at the alliance camp. Guards at the cathedral are 70 elites, also out of my reach. Due to both of these factors, I was unable to gather any alliance-related data.

Horde: Killing guards at the bullwark brought about a few horde players, who just watched for a few minutes and then left, without saying anything. Nobody displayed any real amount of interest (unless I was continually killing their quest givers) until the scourge invasion event started. After this, I was able to kill multiple NPC's in the middle of the undercity, in rapid succession. Once again, a few people came by to see what was going on, but nobody said or did anything, except get bored and leave.

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While I doubt any of this information will surprise any of you, some of you may still find it midly interesting. Feel free to comment.