Stabbed in the heart with a long sword, all the way through, and fall to my knees head hung low, and arms to my side hanging down. and then camera pan's out and fades to black with some kinda emotional music playing.
Shoot myself before he or she gets to me. Bitch, I never lost in the first place
But in all seriousness, it depends on the back story. I've seen a few mantas where the villain and hero actually get together, and though up it's sometimes like, "seriously? What the fuck is this?", other times it's REALLY nice
I kill the hero's loved one in front of him then we have a epic battle to the death,then the hero stabs me in the chest
And as the hero walks away look at him saying "killing me wont bring her back"
Then i beed out with a crazed smile on my face
As an evil one, I would get defeated in a way that would oblige or make the hero/heroin to definitely lose somthing so precious or do something so horrible that he/she would madly damn him/her-self for the rest of his/her lifespan -- if not eternity.
Somthing so traumatizing that he/she would hopelessly and definitely turn into a villain one.
What more evil than making the hero/heroin your best successor ?