I've said this before, but here goes: There are a
lot of opportunities to be quite evil in F:NV, they're just not the overt "BWAHAHAHA, I'LL KILL YOU ALL!" type.
For example:
You can complete the monorail quest such that both sides get screwed.
You can short out the reactor at HELIOS-1 (to do this, choose 'Full Region-Emergency Power' in the diversion option screen); while activating ARCHIMEDES II is a more direct method it also screws
you over, this way you can employ the 'slow bleed' method if you decide to go against the NCR while retaining the services of their merchants.
You can offer to help the Powder Gangers in V19 get out from under Cooke's suicidal stand against the NCR, then blow the vault up instead.
You can offer to help a certain pair of addicts in Freeside, and then get them to OD on their cures.
You can set the Followers up with a steady stream of medical supplies, get the Kings and NCR to go to war with each other and use them up, convince the Followers you're their greatest ally ever by donating
more supplies, then support a major faction and get them tossed out of Freeside after getting the key to their safe-house.
You can give the BoS a glimmer of hope by saving their bunker, only to blow it up later at the behest of a major faction. Alternatively: you can get their current Elder fired, and the new guy ruins them in short order.
There's several more, which I can't remember off the top of my head. You don't get massive explosions or permanent blots on the landscape, but you've done some pretty mean stuff nevertheless.