Dr. Evil had one in "Austin Powers." So did the Wicked Witch of the West in "The Wizard of Oz." Now you can have an evil laugh too. Learn to do an evil laugh and sound just as bad as your favorite movie villain.
Watch movies that have famous villains with evil laughs. "Batman" has the Joker. "The Shining" has Jack Torrance. "Silence of the Lambs" has Hannibal Lector. Each villain has his own, unique laugh. Study different types of evil laughs to get an idea of how to create your own. Some are subtle. Some are loud and exaggerated. Some are just plain manic. Decide which type of evil laugh suits you best.
Stand in front of the mirror and watch yourself as you practice. No evil laugh is complete without the appropriate facial expressions. Do the laugh as you stare at yourself. Try to contort your face in different ways as you laugh. Different facial expressions change the evil laugh completely.
Let it all out. Refrain from holding anything back. If you do not know where to begin, do the standard evil-sounding, "Mwah-ha-ha-ha," and go from there. Do impersonations of your favorite movie villains and give them variations to create your own laugh. Play with the volume and speed of the laugh. Both give your evil laugh a different effect.
Practice with friends. Try your laugh in front of your friends and ask for suggestions. Don't be surprised when they start doing an evil laugh along with you. Laughter is contagious, even when it's evil.
Make the evil laugh your own to suit your personality. The evil laughs of movie villains are unforgettable because they are unique to those characters.