You are what you eat. Trief animals have worse middos then Kosher animals (see Talmud Chullin). So eating treif makes you a more wicked person.
Esoterically there is a explanation for this. (see Chida and Kabbalistic sources for a elaboration for this, e.g. Girgulim Neshomos of Reshaim are in treif so a person who eats treif abosrb there souls and becomes like them).