1 I was logged in when it happened--all of a sudden I started getting "server reboot" countdown messages.  Whatever it was took 'em a couple of hours to recover from--either that or it was a couple million people trying to log back in all at once...

2 This sort of behavior is actually evidence against the hypothesis that what we call reality is actually just an elaborate simulation on some huge computer.  If we were just a simulation, we'd periodically have mass die-offs caused by some jerk hacker from the "real world".  Something on the order of, oh, every 65 million simulated years or so... 

3 oh wait...

4 Being fair, this is a different, and IMO much less interesting, bug.  The corrupted blood glitch was an actual hole in the game logic that allowed the plague to spread outside the dungeon; griefers helped it along, but at no point was the game code hacked or altered in any way, and it *could* happen accidentally.

This is, by all accounts I've heard, a hacker who just temporarily stole GM override commands for a while.  A flaw in system security rather than world integrity.

5 Even with that privilege, it's really pretty amazing he was able to do something so vicious. Killing individual characters I can believe. But there was some way of doing a wildcard "kill everything that's moving" in the city? I can't imagine why even the DMs need such a thing.

6 The City of Heroes developers and GMs have a "Kill all NPCs in this zone" power, primarily for testing on the development servers.  However it can also be used if something goes wrong on a live servers, such as thousands of NPCs spawning instead of dozens, or to clear an instanced map when players get stuck by a bugged mission.

However, as far as I know, there is no "Kill all players" power in  City of Heroes (other than the publisher canceling the game).  I recall reading a post by one of the game's founders on how he was playing a low-level character during one of their early Beta tests, and a group of higher-level testers kept trying to get him killed.  He had no GM-ability to affect them directly, so he just caused the game to spawn a number of giant monsters on them...

7

"Adds."  In MMORPG terms, extra monsters that appear mid-fight, either as an ability of another monster, or a scripted event (more guards rush in every 45 seconds, etc.).  Horse up the code, or have players find some exploit, so you get way more than designed.

Spawn them fast enough that the client software can't keep up with the data. (I've been in fights like that.)  Now you can use a "nuke all the monsters", especially if it's a big public zone that won't reset itself when all the players crash out of it.

Add some script-driven "players" farming them for XP and loot (when there aren't enough to crash the clients).  Now you've got a situation where "nuke the whole zone, player or otherwise" is convenient, since such scripts are likely to log themselves out when they get input outside their parameters (death in a "safe" place, private messages from someone who might be a GM...)

(Messing with cheaters in-game is one of my hobbies.  Er, meta-hobbies.)

8 Based on a youtube video I saw, I think it was a single-target kill command; the attacker just had it macro'd, so he could TAB/macro his way through everybody within LOS within a matter of seconds.