Just as the villainous Mr. Ratchett is patently evil, his victim, poor little Daisy Armstrong, is a paragon of innocence and youth. Her age and beauty work together to render her death all the more tragic in the eyes of the Armstrong family and the train passengers.
Take the chauffeur Antonio's description of the girl, for example:
"Why, that little one – she was the delight of the house. Tonio, she called me. And she would sit in the car and pretend to hold the wheel. All the household worshipped her! Even the police came to understand that. Ah, the beautiful little one." (3.8.34)
Just in case you didn't think Mr. Ratchett was an evil guy before, descriptions of Daisy's innocence and purity illuminate this point brilliantly. She is a symbol of all that is good and right with the world, and our heartstrings are appropriately tugged when we realize that Mr. Ratchett kidnapped and murdered her.