So you want to be evil.
The first question you need to ask yourself is: "Are you sure?"
Most fantasy LARPs are meant to be good-against-evil constructs. There is probably room for not-especially-good characters, and even outright dishonest ones. But actually evil characters, well, that's a whole other kettle of fish. If you play evil, the cards will be stacked against you. The rest of the PCs will want to either shun you, or throw you in the dungeon, or execute you. The monsters aren't going to automatically be on your side, either. That troll coming out of the cave doesn't care where you sit on the ideological spectrum; it cares about how tasty you'll be once it gets through all those black clothes and spiky armor. If, knowing the odds are against you, you still want to play an evil character, well, okay. But consider the following.
You're going to have a heck of a time making friends in character. True, evil people (and characters) don't go around wearing "I'm Evil" t-shirts. Or if they do, they don't get away with being evil for long. But if you're truly playing an evil character, eventually folks will figure it out. The good guy PCs will pick up on the fact that you never tell them anything about yourself, or they'll wonder why you're always skulking about, listening in on other people's conversations, or they'll ask awkward questions like, "Where were you when the skeletons attacked us last night?" Even if you manage to put up a good front for a long time, your PC will practically never be able to confide in any of his fellows. He'll never be able to sit 'round the fire and share his hopes for the future with his companions. ("Me, I just want to save enough money so I can build an inn of my own someday. How about you, Zargo the Dark?" "Oh, I hope someday to subvert the king, place a puppet on the throne, and rule the kingdom with an iron fist from the shadows." "Ooookaaaay, well, I gotta go now....") Maybe you'll get lucky and find fellow evildoers (this is a chancy endeavor-how are you going to do this without revealing that you, too, are evil) to share your plans with. But even then, you're going to have to watch your back. You're evil, they're evil; it's just a matter of time before somebody betrays somebody else. This brings us to another point.
You're going to have to screw people over eventually. Sorry, but its part of the whole evil gig. Your PC, as an evil character, doesn't care about other people. So what if you leave your companions to be eaten by trolls? You survived, right? So what if you sold out your friends to the Necromancer for Zombie parts? You made a passle o' money. So what if you had to murder the Paladin everybody likes? She was in your way. That's a big part of what being evil means. Your PC will stop at nothing to achieve his goals. And sooner or later, all the other PCs will stop at nothing to stop you. Screwing people over does not make one's character very popular. And say what you will about keeping the game and the real world separate, this unpopularity can and does bleed over into the real world.
If you play an evil character, you're going to have a heck of a time making friends with your fellow LARPers out of character. Yes, LARPers all tout the importance of separating fantasy from reality and keeping out-of-game stuff out-of-game, but sometimes, just sometimes, LARPers forget you also have to keep in-game stuff in-game. It's not easy for Joe Player to remember Zargo the Dark, who mercilessly murdered the character he spent three years developing, is not the same as you, the guy who plays Zargo the Dark. Sure, the mature thing to do would be to leave the frustration on the LARP field, but you just killed Joe's character. You probably even betrayed him, 'cause, hey, Zargo's evil. How do you reckon Joe's going to be feeling about you? Don't expect him to buy you dinner after the event, that's for sure. This is especially true if your first LARP character is evil. All these folks know about you is how you behave in character (and maybe how you behave at the check-in table). They will see you acting in anti-social ways, being generally mean. They will draw the logical conclusion: you really are a mean person. If you don't care what your fellow LARPers think of you, go ahead and play evil. But LARPers (including you, Zargo the Dark) by nature are sociable folks. You're going to get lonely.
Robert Heinlein cautioned his readers never to try to convince a person by appealing to their better nature, but rather by invoking their self-interest. Up until now, that's what I've been doing: don't play evil-you won't enjoy it. But the closing argument against playing evil PCs must appeal to the players' better nature. There's a reason it's hard to play an evil PC: people won't like you. There's a reason people won't like you: You're wrecking their fun. When your PC steals from someone else's PC, when your PC betrays someone else's PC, when your PC murders someone else's PC, you generally darken their mood. By playing an evil PC, you're basically making it your goal to mess up everybody else's LARP experience. That's kinda mean. Suppose someone murdered the PC you had spent years developing. How would that make you feel? And you want to do that to other people? You might say, "Hey, it's only a game. They should grow up." Well, true, it is only a game. A mature LARPer would be able to take the death of his character in stride. But no matter how mature a LARPer is, he's still going to be sad when his character dies, and more so if his character died in what he feels is a pointless way, i.e. murdered by you. It's an insensitive thing to do, killing someone's character, and make no mistake: sooner or later, if you play evil, you will find yourself in a situation where you either have to kill someone's character or have your own character die. If you take the logical path, you will be wrecking someone's fun. That's mean. Don't be mean.
If you really want the experience of playing an evil character, I suggest you find yourself a group of table toppers who all want to try evil characters and you go and play evil together. At least that way everybody knows what they're getting into.