You can't make this stuff up. They feel if they lay down their weapons that because of "human nature" he will too.
From the article:
" “The Senators feel the best course of action is to remove all weapons from law enforcement and private citizens so no one else gets hurt,” said a Senate communications intern. “When the gunman realizes that nobody else is armed, he will lay down his weapons and turn himself in…. that’s just human nature.” "