If you became a criminal defence lawyer, I can tell you that most of your clients would be ones who have committed crimes. Only a few would be ones who haven't. The work is still fun and you should remember that the standard of evidence is "innocent until proven guilty". More often guilty people go scott free than innocent people getting convicted(happens, but if you are even a little competent, it's almost impossible).

A lot of times the police also concocts stories and twists facts. It's also one of the very few professions where there is a person dedicated to making sure you fail at your job(the opposing counsel). With all this, you have to be very smart and cold. They will use everything against you. And remember, the world is always "grey" and never black and white. You can find dirt on even the likes of Gandhi (racist against blacks and also slept with his 16 year old niece) and Nelson Mandela. No one is "pure" or "good". It's all a spectrum.