What makes a really good villain

Stories need conflict, tension, danger.  This usually means opposition, often but not always, in the form of villain characters.

Getting your readers to really detest a character isn't difficult - it is actually easier than getting them to love a character.  If the character does nasty things, the readers will detest them.  However, the best evil characters are not simple collections of terrible deeds.  Show their emotion even if it is malevolent - perhaps they use verbal violence as well as, or instead of, physical violence.  Bullies make good villains.

Some the best villains are not shadowy figures or cardboard cut-outs, although there are examples in genres such as horror and SF.  If the reader develops some insights in the motivation and psychology of the bad character he/she becomes more credible, and therefore more terrifying.

Some of the classic villain types are:

• Sadist/Bully (deliberately sets out to cause mental or physical suffering), 
• Assassin (kills innocent people for selfish reasons), 
• Usurper (claims position/role to which they are not entitled), 
• Superior type/Snob (puts others down), 
• Self-deluded/Crackpot (acts out dangerous beliefs)

Some traits you may wish to consider for your villains:

• Humour (may be warped or cruel, or simply sardonic)
• Self-Importance/egotism
• Cruelty 
• Vanity
• Charm/sophistication (perhaps a thin veneer which later starts to crack...)
• Intelligence (makes the villain more dangerous)

Occasionally the villain can become a partially sympathetic character - they make evoke pity as well as terror.  This can deepen a story.

Bad characters usually end up getting their just deserts by the end of the story; sometimes their character defects lead to their downfall.  Rarely, a villain may be redeemed or else found not to be really bad after all.

Put time and effort into creating the right villain(s) for your story, and watch how they behave.