A long while ago, a writer came up with a contest for Rules for Superheroes. It was cute, and genre-specific.
But I always like a smart enemy to go with my protagonists. Preferably, someone who thinks ahead. So, I have Rules for Antagonists. Be they killers in murder mysteries, archvillains in comic books, or your basic takeover the world types in science fiction and fantasy, this should be their playbook.
Rule #1: NEVER. TOUCH. THE GIRLFRIEND. It only pisses them off.
Drazen's Corollary: While abducting the significant other / close relation of your archrival in order to lure him/her into your sinister trap, it will only serve to piss them off. Your normally docile hero, who tends towards nonlethal force, will seriously consider thrashing you to within an inch of your life, and--if no sidekicks are there to enforce restraint--might go three inches beyond that.
Parker's Corollary on Drazen: If you MUST lure them anywhere, use the standard busload of children chosen at random--it will typically serve to get the required effect, especially with a note to the local media sources and/or police agencies. Should you do this in a city with a high protagonist population (eg: Marvel's New York), be certain to request the hero of your choice: otherwise you get all of them.
Rule #2. Don't gloat. The amount of time it takes for you to boast, and threaten, and deliver your well-practiced maniacal laughter, someone you overlooked will have unplugged your weapon / unlocked the handcuffs on the prisoners / otherwise screwed up your plans. Gloat after you've won. (Also: See rule 7)
Fletcher's Corollary: Overlook no one. Seriously, if you're going to take over the world / kill someone / unleash a diabolical scheme, no one is unimportant. Witnesses will provide evidence. Just because it's some nobody in the background asking questions doesn't mean there's no threat—people in the background tend to observe a lot.
Goldfinger's Corollary: Never use the line, "Because you are all about to die anyway, I will tell you all about my evil plan." This includes all variations on this theme.
Rule #3: Avoid patterns. No matter how comfortable you are with a certain places / style / or MO, repeating it on a constant loop will only serve to have the authorities find you.
Rule #4: Wear gloves during your crimes. If possible, wear a full body suit to prevent fingerprints, hairs, or skin cells from being left behind.
Grissom's Corollary For Killers: This includes keeping souvenirs.
Rule #5: Don't let ideology govern your tactics. Just because your ideology says that your enemy is inferior doesn't make them stupid.
Tarkin's Corollary. Underestimating your enemies will get you killed.
Thrawn's Corollary on Tarkin: Underestimating your allies will also get you killed.
Rule #6: Suicide bombers have never won a war, or even a battle. It only wastes experienced soldiers. Even disposable foot soldiers are not infinite. Minions do not grow on trees.
Zahn's Corollary: Unless you have a cloning unit for disposable foot-soldiers.
Von Doom's (PhD) Corollary on Zahn: Or you build your own.
Rule #7 (or Rule 2, Expanded): If you have a chance to remove the adversary from the playing field on a permanent basis, do so. Do not lecture them, explain your scheme, or toy with them. Shoot them in the head, and move on.
Stark's Corollary: Also, do not wound them so they can still be useful to you. If they can be useful to you, they can also hurt you.
Ming's Corollary on Stark: Your enemy can be useful to you – dead. Examples are usually better that way.
Rule #8: If you cannot see the hero, worry.
Murphy's Corollary: If you cannot see your adversary, s/he is behind you.
Rule #9: No self destruct mechanisms, unless they are password protected, and require the passwords of at least three of your closest allies. And confirmed by you. Twice.
Rule #10: Plan ahead. You have all the time in the world to unleash your deadly plot on the world, or execute your crime. Take appropriate precautions, and don't assume that everything will go according to plan. If your plan is perfect, you're missing something. Murphy is always right.
Palpatine's Corollary: If you are going to invite your enemies to come into range of your ultimate weapon, make sure that it's finished first.
Rule #11: No time travel. You don't have enough aspirin in the world to deal with the headache it will cause.
Doctor's Corollary: Unless you have a blue box.
Rule #12: Whatever psychological problems you have, make sure that you utilize them well.
Wilson's Corollary: Don't suffer from madness. Enjoy every minute of it.
Rule #13: When your adversary is incapacitated, on the floor, at your mercy, it is not the time to (a) exercise your creativity in designing Rube Goldberg-esque death traps (b) write your monologue or (c) start gloating. The answer is (d) shoot them in the head and move on.