Did the aliens just want to destroy the human technology?

Yes, the aliens were trying to contact their other alien friends using our technology, but what I don't understand is how they basically just didn't want to hurt any innocent, unarmed humans. They'd scan any human, male, female, child, and if they were just standing there without a weapon / gun, they'd let them live.

While if they were holding a gun / weapon, they'd destroy or attempt to destroy. And this also goes with all the other technology on earth. Anything that looked like or resembled a weapon, they'd try to distinguish.

This even applied to that big giant rolling thing that destroyed the city. It would go around demolishing skyscrapers and highways, yet leave that one kid in the baseball park unharmed.

What gives? What did they actually want? Or were the alien's motives meant to be a complete mystery?