To actually come with a laugh that is evil, first get in the mood of evil. Clear your mind and think about the best villain on TV or in a film according to you. Better still, think of the one person you would truly like to seek revenge on, in case you were really the villain. You may like to do something like this to either your unjust and unfair boss, or to a totally annoying sibling who is getting you into problems almost always. Come up with the feeling you would experience if you actually get a chance to get back at them, think up of things you would do if you had the chance.
Place yourself in the front of any mirror and try giving your face some really evil expressions. This is to assist you to be in deeper touch with your role, or in simple words, to warm up your “evil-laugh” muscles. As an example, try curling up your upper lip towards the inside of your mouth to give the look of a crazy or cynical laugh. Try to furrow your eyebrows and squint your eyesight to give a threatening effect. Keep your face really close to the mirror, so that you can concentrate and actually see your expressions and know if they are actually good enough. Whichever look appears the most evil and wicked to you, choose that.
Having done this, laugh in an evil way. Don’t pre-plan it, just make it spontaneous, whatever comes to your mind. Many-a-times, evil laughs are the creepiest and have the best effect if they are natural. When you try to laugh in an evil way, you will also have to make your mind up if you prefer a low-pitched drawn out laugh or rather a high-pitched cynical laugh. If you feel absolutely at loss, and have no idea what to come up with, begin with the fundamental laughs you hear in films just like “Mooo-ha-ha-ha” or “Eeeh-hee-hee-hee”. The “Moo-ha-ha” sound usually accompanies a low-pitched laugh while the “Eeeh-hee-hee” sound is best suited to a high-pitched laugh. But go ahead and try both in different speeds and in both the pitches, for everyone has a unique voice and yours might require something else. Once you get something you are looking for, practice at various speeds, volumes and pitches to perfect yourself.
As you are doing all this while still facing a mirror, accompany your evil laugh with evil gesture to get the best possible evil effect. If you are trying the high-pitched laugh, scrunch up your shoulder blades and rub your hands in an excited way. In case of the low-pitched laugh, stand erect and tilt the head a little bit. This position facilitates the low-pitched sound to come from within you. Do not be hesitant to try anything that comes up spontaneously for it might just be the “ex-factor” you are looking for.
At the end, try it out on your friends. Take feedback about the effect it has on them and improve till you are perfect.