Yeah, I always play the evil guys in wargames, but I don't play what I would consider an "evil" RPG character. The feeling is all very different between wargames and RPGs, probably because of the scale and the target. Wargames are at such a large scale that I feel removed from the action--my Zerglings can mercilessly destroy the enemy, but I never have to take on that role myself. The target is also important--Dark Elves and Dark Eldar are vicious murdering pirates, but I've never played a scenario in which they murder innocents. A wargame pits two forces against each other--soldiers killing soldiers, for standard wartime reasons. I have no interest in a recreation of a Dark Eldar slave raid against a defenseless peasant village.
As for the definition of what is evil and what is not, it's true that it's all very relative. Killing a person and stealing all his stuff is bad, but doing the same to a dragon is usually considered a good thing. And I won't deny that when I first started roleplaying I played a lot of very amoral characters, who would kill and steal just because the option was a available. (Besides, when you're twelve it's much more fun to kill a lone thug with a missile launcher than just to knock him unconscious.) I think that playing the violent psycho is a normal stage in roleplaying, because it's a chance to explore the limits of your imagination and do things you'd never be able to do in real life. After a while, though, playing that kind of character loses its charm, and "I blow up that shopping mall just because I can" gives way to more mature roleplaying. (Though I doubt that any of my RPG characters could be termed "mature"--I tend to go for the crazy weirdos.)
The line between real evil and game evil has to be drawn somewhere, at least for me. I don't swear, and I wouldn't play a character who swears because I would have to swear in order to do it properly. Sex is a similar case, though there are obvious differences--I don't mind playing a womanizer, but I would never play a rapist because I would feel uncomfortable saying "my character rapes the [insert NPC here]." And while I feel that violence has an important place in games (you have to beat up the bad guy, after all), I wouldn't want to play a serial killer.
Let's look at the sealed section of Dragon magazine, which I don't have but I do know some of what it contains. It includes a bunch of stuff regarding Necromancy--maybe a little too detailed and gory for my kids, but I don't imagine I'd have a problem with it. Necromancy happens to be one of my favorite parts of fantasy, for whatever reason, and I tend to be fairly tolerant of it. The magazine also included some rules for "Corrupt" spells, which permanently burn off some of your hit points when you cast them. That's a very intriguing idea, and while I might not want to play a character evil enough to know such spells, I'd certainly put a Corrupt NPC into my game. I don't see how these rules and spells would be too horrible for children to read, but I'd read them first just in case.
The third section in the magazine is about overt sexual necrophilia, and here I draw the line. Perhaps it is my typical American skittishness in regard to sex in the media, but I can't imagine a game in which such a subject would be appropriate. I can't imagine any plot element that could not be accomplished through less distasteful means. I wouldn't use this material, I wouldn't let my children use this material, and I might not even read that chapter of the book. HOWEVER, I don't see any good reason why my personal prejudices in this case should be forced onto other people. I wouldn't want to ban the book just because it contains stuff I don't agree with. As an adult I can make my own decision about whether or not to use the material, and I afford other adults the same right.
Rating games and "carding" the purchase of Mature games is an interesting solution. I support carding because it only affects minors, which means that a) it doesn't hurt me, and b) it leaves parents the option of investigating the material and buying it for their children if they deem it appropriate. Any guidelines stricter than that would be unfair--it would limit adults in the name of protecting children. It doesn't make any sense. Spriggan has an excellent point, however, when he says that most RPG stores are too small to bother with carding. Only 15% of video games in the U.S. are purchased without adult supervision, but I'd bet the numbers are reversed when it comes to RPGs.