But Lucius Malfoy didn't know that he was holding a bit of his master's soul in his hands! He wouldn't have thrown it away to some girl if he'd known it was a horocux.
But there was Lucius, having an old book Voldemort hade left him, and he just thought it was a magic object, full of dark magic.
So, he thought that if Arthur Weasley's child was found with a thing like that, he would be fired from the Ministry. And Lucius wanted to punish the Weasleys, because they were truck with muggles. And he hated them.
Poor Lucius, Voldemort couldn't have been happy when he found out that one of his horocuxes hade been destroyed