Chris is equating "practical" with "self-interest." His father didn't accept blame for shipping out faulty engine parts out of self-interest. It would cost him his job. Chris never faced the possibility that his father was guilty because it would have complicated his own life. His mother did the same thing. And so he avoided thinking about it so he would not have to take action.
Chris is comparing what he did to what he should have done. He recognizes that he has a higher responsibility, not just to his family, but also to the soldiers. As an officer in the army he accepted this responsibility. He is troubled by the realization that his father had a responsibility only to himself and his family. He was practical.
Chris has to make the decision to put aside being "practical" and do what he has to do, despite pressure from his family.
As Arthur Miller's play All My Sons nears its finale, the once-loyal-to-a-fault son of a father whose actions betrayed everyone and everything he holds dear finally begins to wipe the veil from his eyes. Joe Keller has lived a comfortable life, having profited handsomely from the military contracts his business had won during World War II. Throughout Miller's play, however, a darkness has loomed over the Keller family, and that darkness emanates from Joe's duplicity and the death of his other son, a pilot during the war who could no longer live with the knowledge that the aircraft parts Joe's company produced were defective, resulting in the deaths of other American military pilots. Chris Keller, heretofore, has been the dutiful, loyal son, defending his father whenever the subject of the deaths and Joe's possible complicity comes up. And, as All My Sons progresses, the subject come up a lot.
Once Joe's guilt--a guilt that includes not only the deaths of American military personnel and of his own son, but the imprisonment of his former employee, the father of Ann Deever, Chris's girlfriend and the former girlfriend of Chris's deceased brother Larry--becomes too obvious to ignore, Chris finally abandons his former sense of loyalty to his father and announces to his mother, Kate, that he intends to leave town forever. It is in this context that Chris states that he has become a practical individual:
Chris: Mother... I'm going away. There are a couple of firms in Cleveland, I think I can get a place. I mean, I'm going way for good. (To Ann alone) I know what you're thinking, Annie. It's true. I'm yellow. I was made yellow in this house because I suspected my father and I did nothing about it, but if I know that night when I came home what I know now, he'd be in the district attorney's office by this time, and I'd have brought him there. Now if I look at him, all I'm able to do is cry.
Mother: What are you talking about? What else can you do?
Chris: I could jail him! I could jail him, if I were human any more. But I'm like everybody else now. I'm practical now. You made me practical.
With this exchange, Chris is conceding that the only way to survive in the world is by being cynical and by ignoring the crimes to which he now bears witness. Whereas Kate has long known of Joe's guilt, Chris has allowed himself a certain degree of self-delusion regarding his father's character. Ann's revelation of Larry's letter declaring his intention to kill himself rather than live with the knowledge of Joe's crimes destroys Chris's ability to continue to deny the realities that surround him. Unsurprisingly, Chris cannot bring himself to report his father to the authorities, leaving him only one viable option: leaving home and never returning. It is, he concludes, the practical course of action.
Chris know is practical unlike himself when he first came back from the war, he was idealistic (because he could not live his live again because he saw his men dying for each other in battle). At that time Chris could jail his father if he knew that he was the criminal, but after he stayed three and half years in joe's house and used his money he made himself practical, so he could not jail his father. Chris's morals have been changed.