Early in the game, I can cause the destruction of an entire town. Megaton will be scarred for hundreds of years. I poisoned the water supply which will basically kill every settler in the wasteland. I annihilated the Brotherhood of Steel and removed them as a threat in the Wasteland by dropping an orbital strike and decimating it. This was very brutal and had a large-scale impact, mainly being by large explosions.
There are evil decisions to be made in NV, but I have not come across any options to throw nukes at towns, poison water supplies or be able to cause a Holocaust without having to resort to weapons.
(If there is any Megaton/Water poisoning holocaust-ish things to do, please post it without trying to spoil it entirely, but lead me in the direction please
Another sad thing is the removal of keys on dead merchants. I loved visiting merchants I deemed not worthy of life, smiting them into oblivion, take their keys and loot the Brahmin/stash locker. I can understand removing the keys from the living person because stealthy people could do this, but if you murder somebody, what use do they have of their inventory?
Thanks for listening.
PS, on the flipside, I have noticed in NV compared to F3, there are some items that are owned by evil people but still make you suffer karma losses. In F3, if you stole from a mean son of a [censored], nothing happened. In NV, if you do it, you lose karma regardless. It is called Karma for a reason! If you make a living killing and stealing, you should EXPECT somebody to steal/kill from you!(killing you can do, stealing can't)
Thanks again for listening.