Case in point: there’s a new sci fi TV show called Falling Skies.  Simple plot: aliens have landed in Massachusetts (and other locations) killing adults and abducting children.  The aliens attach large slugs to the backs of the kids in order to enslave their minds.

One day, a man just shows up in the encampment of people trying to stay alive and kick alien butt.  He’s known
only vaguely to the military leader of the group. He states:

“The Skitters (aliens) are coming.  They’re almost here.  I’ll protect  your children by taking them north for a couple of days until you kill the Skitters or drive them back.”

There’s nothing ominous in what he just said.  However, why do you, the viewer, instantly know he’s in cahoots with the aliens?  Because of the music.  You know, the old  “dah … dah … dah … dahhhhhh” type of thing.

However, as a writer, you don’t have the luxury of menacing music.  You have to get across his evil intent with words.  See how this reads:

“My group was attacked by the Skitters and I was the only one who escaped.  They’re headed in your direction, and will be here in less than a day.  Your children are vulnerable, and you know how difficult it’ll be to fight the Skitters while trying to protect them.  I’d like to help you out, so how about I take the kids with me; we’ll head north for a couple of days, and when you’ve killed the Skitters or have driven them back, I’ll bring them back to you.”

The first sentence instantly raises my paranoid antenna.  How coincidental that he was the only one who got away.  Nevertheless, that could be true.  But how did he just stumble upon this particular group?  How does he know in which direction the aliens are headed when he ran away while his group was being slaughtered?

Why is he being so willing to help them out, especially if he doesn’t know them?  He plays on their emotions, reminding them (as if they needed to be reminded) how hard it is to fight the aliens and keep their children safe.

He immediately follows it up with a plan that will separate the children from the adults, in the guise of keeping them safe.  Why does he plan to head north?  If he doesn’t know where the aliens are coming from, they could just as easily be heading south, where they could intercept him and the children.

And—here’s the kicker—how will he know when the aliens have been vanquished and it’s safe to bring the children back?

In order to lend this a sinister air, you need to create uneasiness in the readers’ mind as you do in the mind of the characters.  The reader needs to be able to picture how this good samaritan comes across to the adults, who, understandably, are not sure they want to be parted from their offspring.  It may not be obvious to everyone, but it certainly raises some doubt in the minds of several parents.  Who is he?  Why is he being so helpful to strangers, at the risk of his own life?

Dah … Dah … Dah … Dahhhhhhhh.