I dunno, offhand I can think of one thing.  I'm going from old memory here, and I did mention it once on the forums, so forgive me if the details are a little off.

  I was on the tail end of a game and going for a diplomatic victory, but the Altarians simply could not be brought to my side.  I think it had something to do with years of me capturing minor race homeworlds in Altarian space and using them as influence bombs to steal their planets.  After lavishing gifts on them only to see relations rise then immediately drop back down to wary/hostile, I could take no more.  Outright war was out of the question.  The Drath and Torians would leave my alliance and side with the Altarians.  I needed something else, something that would go beyond simply conquering them.  My frustration with their intransigence had boiled over into unbridled fury, and they would pay a terrible price.

  So I began long preparations to crush the Altarian Resistance.  Multitudes of transports and constructors were built and moved into position near key Altarian systems.  Soon, Terror Stars were under construction throughout their space.  When the time came, I provoked them buy floating a few transports past their planets.  They called up to declare war as retaliation for my actions, rushing to their own doom.  In six weeks of war, they lost about 25 of their 45 worlds, 9 of which were destroyed by Terror stars, including Altaria itself.  During our last negotiations, they ceded their remaining worlds in exchange for the lives of their people.

The Altarian Resistance was no more, and the remaining independent powers united under my leadership.

My meanest act might be what I did to win my latest game.

There is this minor race who stole all of the technologies of the most technologically advanced major race via an event.

First, I declared war on the minor race.

Then I blockaded the planet and smashed all of their ships.

Then, I forced them to pay up every last technology and credit for peace.

Then, right after I made peace, I declared war again and conquered their planet! 

The technology boost is enough for me to overcome my enemies!

Mine dates back to Civ I. I played many games where I built the Manhattan Project mainly to add the piddly graphic to a single city's image, and I dutifully built a 'deterrent' infrastructure because I (still) have a vague respect for the MAD doctrine. But on one sad day, a stupid AI lobbed one of its scant handful of nukes at my realm and we retaliated by nuking every major city in that AI civ. 

Cocktails and college class deadlines were involved in the decision-making, but the heart of the matter was some helpless talking/naked ape shit. The game surprised the shit out of me by being so stupid about nukes, and I responded with equal stupidity about nukes. If Civ I had included even limited climate change models, my brutal response would have meant the end of the game for all players, but alas I 'won' that game.

I think I can top these as far as pure evil goes.  Mine goes back to the old days before the internet, where we'd play multiplayer games on BBS's.  It was a space game called "Tradewars" where you had a certain amount of moves (1 move per tile on a grid) and then had to wait until the next day to continue.  You'd normally have up to 200 moves, but if you died, you'd be limited to 50 so you could escape.

I was aligned evil (of course) and was a member of a 5 man Corporate (team).  The #1 player was aligned good and had incredible weaponry on his ship.  Normal ships in this game would auto retreat if attacked.  This guy had an auto attack system and we had a scuffle above his planet.

Mine was the most powerful "pirate" ship, but it was no match for his, even though he was unattended.  I couldn't win the battle toe to toe, but I stayed in the fight and mined his planetary system with every mine I had before he destroyed my ship.  My mines had damaged his ship, but he was still alive.  

I was in an escape pod and still had turns left, so I went back to my Corporate's planet and borrowed a teammate's freighter.  I borrowed every mine my teammates had and went back to the "good" planet, dropped the mines and went back home, leaving a message for my teammates before logging off.

Later that night, he logged on and immediately blew up, having activated the mines.  His logs showed me losing the fight and then returning, so he accused me of cheating, but I had no way of cheating and wouldn't if I could.  It was a pretty funny message that he sent and he posted it publicly so everyone could read it.  We had a good laugh.

The evil part of this is that I not only didn't gain from his destruction, it cost me turns that I could have used to rebuild myself.  I also could have retreated when we first met and would have survived with my pirate ship intact.  So I did myself "harm" by causing his demise.  

It was so worth it.    

Hoarded over a quadrillion credits.

Now that's being tight-fisted!

For me, it wasn't in GalCiv2 but in Star Empire 4...

Even if it's an old game, I still enjoy to play it from time to time... You know, in this game you can get your ennemies's planets by forcing them to rebel or by making the owner surrender to you.

The problem is that you get the planets with their entire population and, even if you assilmilate them in your empire, they keep their caracteristics (type of breathable atmosphere and so on).

So, systematically, when I get such planets, I dump all the population (youngs, olds, males, females, pets...) into space and replace them by representants of my race.

Sometimes, when playing GalCiv2, I regret I can't do the same.... But, nethertheless, it's a great big game.