"I believe people pay for what they do, and still more for what they have allowed themselves to become. and they pay for it simply by lives they lead." In this context "pay for what they do" means to suffer a loss, or a consequence because of something they did. If you steal someone's car you will be arrested and put in jail. You will "pay" for stealing the car. The phrase "for what they have allowed themselves to become" is similar but not so specific. In this phrase they aren't talking about one example, like stealing someone's car. They are talking about the sort of life that people end up having. For example, if you really like money you will spend all your time working and very little time with your friends and family. You may even involve your friends in bad business deals so that you can profit from their losses. After years of doing this you will "have allowed yourself to become" a bad person who only thinks about money. Perhaps you will have lost all your friends and even your family will not want to speak with you. If you steal a car your punishment is going to jail. If you only care about money your punishment could be living your life without family or friends.

Thank you, bj reeves~

I think your interpretation of ‘karma’ and the boomerang is spot on. The phrase about what they have allowed themselves to become’ is poor sentence probably just referencing her negative judgement of someone.