'I'm a good person and i didn't harm anyone' for me this statement is very subjective... for one person it could just mean i do nothing so i didn't harm anyone therefore i'm a good person but for another person it could be i hold the vows and didn't break my vows therefore i'm a good person.. or it could be i not hold vows but i only help other and i never harm people therefore i'm a good person..
i understand in this case is a first one that they do nothing and they didn't harm other therefore i'm a good person.. i believe that most of us are like that before we met buddhism and learn and understand because we are raise up in that way from our parents. But why we changed our thought and willing to do more? For some people because they understand that not to harm people is not good enough we should do thing that benefit other as much as we can. For some people because they saw someone do it and it click to their mind and it inspire them to do the same even not really understand the benefit..
so to answer the question, there are so many things to do to change their mind either by education, talk, explanation, show sample, let them feel and experience, and how our actions, how we treat people etc and it also depend on the how open of their mind to accept it.. hence to nature someone it depend on their believe and faith and the method we use with them with each person are different due to their karma ..