We sit at the keyboard, contemplating the story in front of us. It needs a hook. Something that will really reach out and grab the reader/publisher/agent by the throat. So we start typing.
Thus the most horrendous of all evil doers is born.
There are many ways to destroy a story. Take the villain mentioned last time. S/he/it eats babies for breakfast. That's over kill in characterization. What is scarier – a raving madman or an intelligent, soft spoken antagonist who is perfectly sane?
For example, I see people on TV all the time that scare the hell out of me. Turn on CNN any day, if you can't find the perfect antagonist, you aren't paying attention. They are in politics. Most of them have their own "News" programs. Their opinions are as twisted as a bag of snakes, broadcast for the world to see. Don't listen to whatever/whomever they are bashing; look at the person doing the bashing. I shudder when I think that the world knows "us" by these vultures.
Granted, no one would take a villain based on Rush Lumbaugh seriously, except in a story of the most over-the-top the horror genre. But there are others, who look and act saner that illustrate the point.
Overkill isn't limited to 'the world's most villainous villain' either. Purple prose will turn off a reader, no matter how well intentioned. If a 'hard-bitten hero' (already treading the line with this guy) does a sudden about face – say starts spouting poetic gibberish to the heroine he hated two seconds before… Well that does it, overkill strikes again.
The root cause of most blatant overkill is writer insecurity. "Will they get that?" The writer wonders, chewing fingernails, then decides to play the situation up even more. The result is a massive overkill that stops the story in its tracks. The hero is heroic to the point of suicide, the heroine too helpless/pretty/angry. The villain – well – eating babies for breakfast is the warm up, the start of her/his/its day.
Your villain does not need a boat load of torture equipment to be scary. Your heroine does not need to have sex four times a day with multiple partners to be sexy (or slutty.) Your characters do not have to escape every battle unscathed.
You don't have to underestimate the power of the written word.