No more understanding.......

As a child growing up in a Christian home and having both parents as ministers in a church I was taught that by nature people are good and sometimes they got lost in their circumstances. I've come to the realization recently that yes, most people are by nature good if you are willing to search for it, however, there are some people that are not. They are bad people, they are selfish and self-serving. For the last few months, I have searched and I have tried to find good in a particular person, even when I didn't want to....Even though I knew it would be so much easier to hate the person, I just kept saying "this person has had it rough, they need patience and understanding more than ever now"...Even though they have said some horrible things about the people I love and have tried to break me.
Some people will always choose to cause everyone around them misery because they are unable to find happiness themselves, they are blind to the fact that it is themselves that are causing the true unhappiness. You try to stick up for them, saying that it's not their fault that they are acting like this, they are hurting, they are scared--but maybe that's just a copout, perhaps the person is just selfish and undeserving of anything that anyone has to offer to them. How do you find love and understanding for people like this? When do you say, enough is enough? I have no more in me to give, I have reached that point now...I feel naive to believe that by being patient and caring this person might find peace.