I guess what I mean to get at is, you can't really claim that online you have moral choices beyond the usual dickish behaviour we see in our online games. You can't decide to be a pacifist and level up just by telling people to make love, not war (certain exceptions of course apply). Nor can you decide to use the tactics of truly evil men, and perform genocide, or level cities (again, of course, certain exceptions apply). If there is the ability to do one of these things, you are almost guaranteed to never see the opposite side of the spectrum within the same game.
Certainly virtual worlds have more freedom for morality (albeit only a small amount), than say a console game. Because within those games you are only ever affecting yourself (multi-player games aside), and thus the games reactions were already thought of, planned out, and set in place. Anything you can do, the game knows, anything you can't do, is because the game will not allow it. In MMOs there are things that the game will allow that its designers may not have thought of, but you can be certain if it is truly against their beliefs they will simply patch it.
The reality behind it all, is that aside from personal choices to try and modify your playing experiences while being forced to stay within the confines of the game rules, you are not allowed to do anything the game designers and GMs do not want you to do. When you act out in chat, perhaps in a fashion not befitting a pillar of the community you are often put on a temporary suspension, or perhaps even a permanent ban. When you MPK other players, or grief on certain server types often the same sorts of things apply.
If you spend hours wiping out the entire populace of a certain monster or people's village they do not stay dead, they simply re-spawn a few minutes later fresh and new. No one mourns their deaths, no force pulls you off to a war crimes tribunal, the world is not appalled at your actions. Quite the contrary, people often laugh, and praise you for your newly found +3 stick of water making that the four thousandth creature dropped.
True morality cannot exist without consequence, and thus is the main problem with any virtual world. Aside from a ban from the game, in which case you can easily pop down to your local gameporium and just pick up another subscription (if you are really that dedicated a griefer).
The only game I'm aware of that can really foster a truly malicious attitude would be EVE Online, where a group of individuals could form a cartel and truly rule the universe with an iron fist, killing any and every person possible, for whatever means. But even then, they still must do it all within the confines of the rules laid out by the game designers. They cannot decide to force people into slave labour (because that person can just log off, or complain), they cannot make people give up their homes and families. They cannot find new ways to kill or steal by altering the game (hacking/exploiting/etc). They must use the tools of evil that the developers chose to put in the game, and those tools only, effectively removing any choice and free will they may have had. A tame, collared and leashed type of evil. You could argue that the same sorts of ideas exist in reality, that with our laws and governments that there could truly be no perfectly "evil" organization or person. And while that is historically true (mostly), you could then counter-point that there simply hasn't been a force strong enough to topple every government and take over themselves.
I suppose at the end of the day it's probably for the best that true 'mmorality' and choice does not exist in online games, or it wouldn't take too long for one faction or another to control a server, or a game in its entirety. I just wish that game designers would stop throwing around this idea of freedom of choice and morality like it's something that defines the game. Your laws are no different than the laws of your predecessors, you simply shined them up a bit, and made a big stink about it.
I could really ramble on about this topic for a very long time, like others before me, but I can only do so much with 1000 words and 1 pot of tea.