Unfortunately, replies to the "How could you?" question that have a Constitutional dimension aren't personally satisfying to the questioner. Just because every defendant has the right to an attorney doesn't mean that you, personally, have to choose to be that attorney. Such a lecture is bound to give the impression that you are either a mincing shyster hiding behind a technicality or a high-minded idealist with little experience as to how your career affects "real people."

I've always found the following much more effective: "Well, I want to work in criminal law because it personally interests me. Unfortunately, you only have two options: to be on the defense or the prosecution. All things considered, I'd rather defend guilty people than prosecute innocent ones." This stops most people in their tracks. Once they realize that justice is a double-edged sword, most people (rightly) begin to see that your personal decision as to where to locate yourself within the system isn't something they can morally criticize on any generalized basis.