How does Miller makes the audience to sympathise and despise with Keller's character ?

Keller is his own worst enemy, thus, we come to sympathize with him. Initially, we see Keller as a man who sold his friend down the river, a man who knowingly put young soldiers lives at risk. Keller is selfish, but he's not unfeeling. His guilt affects his relationships with those around him, the revelation and acceptance of his guilt brings him to his knees.