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To me, too many games have stereotypical villains. A knight in black armor decorated with skull motives. A black robed wizard with a staff with a human skull on it. YAWN. The typical villain sacrifices babies and tortures/mutilates the innocent and is a nice basic BBEG (Big Bad Evil Guy/Girl) and the game can run fine but I prefer more detailed villain.
One thing I don't like found in a lot of adventures is evil's fascination with the undead. Zombies smell. Would you want them in your castle while you dream of power and glory? No, and neither should the majority of villains in your game. To me undeath represents a final line that the most evil of aligned beings don't cross. Would you really want to be an animated corpse even if you get to keep your sentience? I see no reason why most rational beings would go there when other more mundane options are available.
There's also a reason why half fiends should be rare. Mating with a succubus isn't much stretch of the imagination. However if you have any minis from the D&D line, compare the size of a human size mini to a Glabrezu. Mating with an elephant sized dog faced beast from the stuff of nightmares shouldn't be something even evil doers would consider. True, a good villain can be insane but the ol' "I'm CE and insane" villain gets repetitive quite fast.
In 2nd ed one of my players had a priest that more or less had to follow the Paladin's code and worshiped a LG sun god. On one of his forays around town he came across an attractive noblewomen. One conversation and a detect evil spell later with her she admitted to following a CE god opposed to his own god. He was ready to unleash holy wrath right there and then on her but she refused to fight! The player even tracked down where she lived and went and visited her occasionally. The priestess invited him in and made him feel welcome and supplied food and drink (after a detect poison spell). The rogue snuck in and couldn't find any hidden alters or murals of demons eating human that usually adorn most fantasy villains lairs. Yes, she was CE, impulsive and selfish but she knew she couldn't defeat the LG cleric in a fair fight and had no incentive to look for trouble with him. She was sane and didn't sacrifice sentients to her god. She was much more effective as an infiltrator for her religion. If I recall they eventually had a battle when he found her in a temple but at that point they couldn't quite bring themselves to kill each other--by then they were familiar with each other and almost friends, almost.
Remember evil beings have feelings too and there's no reason why they can't have interests besides destroying the world or can't do good deeds. The concept of an anti hero is an old one and one of my favourites is Elric of Melinbone. This tragic figure of fantasy destroyed his kingdom, killed the women he loved, and more or less destroyed the world. Why would an evil aligned being do good deeds though? Because if they're not one-dimensional they have feelings, interests and loved ones. If a CE sorcerer came home and found out that orcs had killed his children odds are he won't be too happy with them even though they're both CE. Or goblins for a LE character. Perhaps they want to save the world so they can dominate it or because their homeland is under attack. Yes, evil can be patriotic as well. Love can motivate an evil character as can racism.
While most people believe love is reserved more good and neutral alignments there's no reason why evil can't love as well. Dragons, in particular, protect their young and while not true love several evil races defend their young. Racial pride and reproducing the species should motivate some beings, if nothing else. Even evil people often have friends and family they can care about. An evil family is probably more trustworthy than an evil stranger. Another aspect of fantasy literature involves an evil character falling in love with a good aligned being. Why mix fire and water? Using an assassin who falls in love with a Paladin for example could be interesting. The assassin might think the Paladin is a bit foolish but at least the Paladin is trustworthy. Of course the Paladin may or may not feel the same way in return. Evil/good beings may just plain old be attracted to each other.
I'll end my ramble here but evil is an interesting alignment. Give a villain a weakness or a virtue. Make them have a soft spot for children or have them donate money to a charity or something. Heck once a villain is defeated perhaps the heroes can deal with his distraught but innocent family or find their now orphaned children out back.
Al's Notes: A real life example to the above comes from the 2nd world war where the most notorious of evil men in the 20th century existed, Adolf Hitler. He had a girlfriend, which he married at the end of the war, and adored and entertained children. He also took care of his secretaries quite well. All this dispite being one of the most evil man in our history.