What reasons does Mitch have for abandoning Blanche? Is it justifiable?

Mitch is attracted to Blanche from the start, and Blanche hopes that he will ask her to marry him. Indeed, Mitch is a fundamentally decent man and seeks only to settle down. But when the truth about Blanche's history comes to light, he feels swindled by her. These feelings of betrayal lead him to abandon her. When the doctor and matron appear, Mitch can't even look at her, and though he is angry with Stanley and the two come to blows, he never intervenes.