I guess the geck was needed to clear the radiation from millions of gallons of water quickly. I could accept that. After giant mutant critters, mutated people, and ghouls, it falls under the SCIENCE!! rule for me. Tons and tons of reinforced concrete rubble disappearing/recycled/whatever the geck does with it would have been much harder to accept.
And your actions not mattering is why I preferred the "game ends" ending of Fallout 3. The game was supposed to end and had a closing slide with narration for what happened if you used the fev. The slide showed you a dead super mutant, the inhabitants of Underworld dead, and the inhabitants of Rivet City dead. Truthfully I think everyone not born in a vault would end up dead, and that includes the player character. Eden didn't know you weren't born in the vault; plus the choices you make in game that add a couple of benign mutations to your character means that your character would have died eventually too. Not to mention the animals; just going by the looks of them, I don't think the brahmin would last long with the fev in the water. If there are any people left, you just wiped out one of the food animals they rely on. "Everybody dies [except the still closed vaults and the Enclave]" does appear to be the "evil" ending.
See that's what I'd prefer to avoid with Fallout: New Vegas. Don't get me wrong, I did enjoy Fallout 3 very much, but alot of the story made zero sense to me and some of it made even less sense after the Broken Steel DLC.
As for the G.E.C.K. I just subscribe to what http://fallout.wikia.com/wiki/Vault_City demonstrated in Fallout 2. I understand that there may be other variables at play, but if Vault City could manage to carve out a idyllic settlement in the middle of the wasteland, why couldn't the scientists in Rivet City do that? Wouldn't that be a better option instead of just clean water? Wouldn't you be able to use salvaged Vault technology to purify water? What about recycling urine into water (I know it sounds gross but it's feasible)? Why was destroying such a useful piece of technology the only option?
Regarding the evil path I heard a good point from another forum dweller in another thread... The gist of what he said being:
The evil path comes out of nowhere. You spend all that time working up to finding the G.E.C.K., then get captured. The only 'evil' thing you can do in the main quest really is accept and use the Modified FEV. So basically the Lone Wanderer is a straight shooter until he meets President Eden... Only then does he really have a moral choice, which are two extremes: Turn on Project Purity and give clean water to the masses
or contaminate the purified water with a deadly poison that kills any ghouls, super mutants, and over-exposed people. Why such a huge leap from good to downright evil?
Whew... Got it all out there.