I guess I didn't make a proper answer to this question yet, so here goes a big one(brief conclusion on its ending for those who don't like to read much):
I simply love the idea "Empire-building" games. The prospect of starting with almost nothing, and to work all the way towards the construction of a millenia-standing world or galactic power has an epic feeling(and length - not reccommended to play 30 minutes before going to school/work/etc.
) that is truly rewarding for the gamer who struggles against the difficulties of such genre. Alas, such games can also be source of inspiration for many histories
[1], specially when themselves have the differential of a strong plot and deeply developed ideas of factions/civilizations/races with them. Imagine in Civilization, how would be the world reaction when the wonder "Cure For Cancer" was developed by you, the lifes saved, now imagine equally the fear your name draws as you conquer the world and finishes "Project Manhattan". Unfortunately, it never gave much depth on the consequences of your actions, people who died of starvation were just less human faces on a city management screen, rebels are just angry faces on the same screen, the maximum that could be seems was the ominous mushroom effect of a nuclear explosion against a city and the ruined houses of invaded cities in the "View City" and "Zoom" modes of later Civ games. The best way to show the results of your acts was through FMVs, something where the game Alpha Centauri truly shined.
Now, you would probably ask, what does "Empire Building" have to do with "Evil" characters?
Power(political, social, etc.) and the struggle for it are common souces of many atrocious types of "evil", of unimaginable acts of terror and therefore is not a bad primary motivation for the life of a selfish, "evil" character. The ambitious search of domain over others is a path many times marked with the blood and suffering of the innocents, and if at least the average human being of Yliakum is akin on its essential nature to Earth's humans,
Milgram experiment shows what power could accomplish.
The major and hardest struggle is what draws me more interest in any history, in a dystopian Setting, it would be creating an Utopia and fighting against "evil", in an Setting with a strong establishment of values, with a neither dystopian nor utopian situation(relative peace, etc), like Yliakum, it would be either taking over control of it for selfish goals or turning it into an true Utopia, because when things are "good enough" most accomodate themselves and accept the way things are. In this case, becoming a significant source of power, on all its spheres, is a very difficult goal and usually the eternal goal of the "Evil". When there is no more villains to fight against, the "hero" will settle down, while when the "evil" ambitious individual seeking for power and for the establishment of his/her uniques values conquers and shapes a place/group of people/nation, he will not settle down, but instead continue his search for power to expand over continents, worlds, the solar system, galaxies...*I would have to type infinitely to describe more things*. So the struggle of "evil" is eternal and challenging in a place like Yliakum, while the one of "good" is temporary and relatively easy in a place like Yliakum, except if this "good" character sees government as evil, then its goal will be equally very difficult and eternal as well.
Now going back to the reference of "Empire-building" games, it's well know the game becomes uninteresting for the peaceful(in the sense they only use military force to defend themselves in most cases) "builder" types when you already are the uncontested world superpower and the opposition has any chance of defeating you. The same happens for "good"(and specially "lawful good") characters in Yliakum, the difference being that the chances for the other side are not 0%, but still low, yet this makes the game somewhat uninteresting if you are looking to do more In-character than to RP an average citizen or if you do not want a 100% peaceful roleplay, because the difference between a settled "hero" and an average citizen is very small on their daily lives. This is why my characters are either "evil" or anti-establishment on a certain degree.
Concluding, roleplaying on the side of the "underdog" of anything is something that not only is more interesting, but also more challenging, specially if the struggle is more permanent than temporary. Seeking peace and freedom in a world dominated by war and tyranny or seeking power and strife(or Anarchy on
one of its meanings) in a world dominated by fairness, "democracy"and peace sounds more fun than going with the flock and eliminating the few existing "foes" easily, also by opposing the "mainstream" you are creating conflict, and conflict is one of the elements that give a new impulse to any history(or roleplay). Also the quest for achieving power is a very interesting path for a character(and its player) that is usually filled with morally ambiguous types.
[1] Here a forum with a list of "roleplays"(histories based on gameplay) created with the "Empire-building" game Sid Meier's Alpha Centauri, as an example:
http://apolyton.net/forums/showthread.php?cache=1&threadid=133871