Integrity, why do people sacrifice theirs?

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I have and acquaintance, a former friend actually who did something deeply offensive to me about a year ago who now is doing everything in his power to undermine me personally and professionally. He knows that what he did back then was wrong. I even took all of the steps to make things right between us again. Still he continues to meddle in my business. I don't care what he says personally, because the people he is saying it to know the real me. What he is doing to me professionally and in other areas of my life is making things impossible. Why must people sacrifice their integrity for personal gain?