It may appear at times that the conscience is easily damaged; but I think that is an illusion that implies darkness is more powerful than light. I think the conscience is our connection with God. It is how we are like God, made in His image and likeness. Thus it is not easily overpowered in a way that damages it totally.
The Bible talks about the heart in a way that conceives of it as having pieces. It speaks of hard clay that is dead and brittle and soft clay that is alive human. If the heart has brittle pieces, it needs circumcising. The hard pieces can be cut out and replaced with the soft yielding heart that responds to love and reality.
I think of the conscience mathematically. Suppose you start off with your heart having a thousand soft pieces and you harden one. Your good impulses still outnumber your bad ones 99:1. The odds are with you then that you'll correct the bad one. If you harden another piece, the ratio goes to 98:2 or 49:1. The more wrong decisions you make by defying your conscience, the more the heart is hardened and the more corrupt it is with the evil urges assuming more influence. If you reach the point where the evil is more than the good, the odds really are that by yourself you will not be able to correct yourself on your own. You need help. If you accept help, the good in the other person can work with the good in you to master the evil.
If we take on Jesus as master, he combines his good with whatever good we have; and that is always enough to master the evil. I think most people are born in the 40% to 60% range. The better shape you are when you accept Jesus, the easier things will be, the faster things can go. The worse shape you're in, the more bumps there can be in the road and the risk of rebelling against Jesus rises (as in the parable of the wheat which sprouts by then dies quickly).
Even the most wicked of people can change for the better . . . if he is willing to allow the good in Jesus to work in him; but the harder the heart, the harder it is for that person to change. There comes a time when theoretically it is possible for someone to change but the odds are stacked against it. You could predict failure and almost always be right. You could be wrong, but you probably wouldn't be. Thus it was safe enough to predict that Pharaoh would harden his heart even more than he had in the past. Pharaoh got pressed hard but refused to allow his heart of hard clay to be replaced by soft. When pressed hard, he'd say something good and make promises; but once he thought the pressure might be off him, he'd go back on his word. He never genuinely repented, only pretended.
There is a type of person Jesus compared to pigs. Don't give them pearls since they won't appreciate them. They'll turn on you and attack you. It's a good rule. If you do good for someone and he repays you with evil, odds are he's a pig. You're wasting time on him as well as putting yourself in some awkward situations that might damage you.
Here's the philosophy I think the pigs have. They believe everyone is probably evil -- and that if you do something kind for them, obviously you have some hidden agenda to take advantage of them. Or they interpret your kindness to them as weakness meant to tell them you're weak and don't mean to be a threat -- but if you had the power, you would show your evil. The person who thinks like this cannot be helped by others being kind to him. He just returns evil for good -- making his situation worse, hardening his heart further. The best thing to happen to people who live like pigs is to let them suffer. Like the Prodigal Son, perhaps after they've suffered enough living like pigs, they'll come to their senses and repent of their craziness.
I advise studying people's reactions. I think most people, even those with sour attitudes, respond well when you return good to them for evil. They say to themselves, "Maybe the world isn't all bad. Maybe there are good people out there. Maybe I too can be good." So they respond back with good. Always try the good for evil approach first. Study the responses, and if someone gets worse, he's not worth persevering with. The wasted effort you put in with him probably could have been spent on ten other people who will respond favorably.
If you can see someone is acting like a pig and keep throwing pearls in front of him while he continues to rend you, you may wind up discouraged about the power of Divine Love to change people. It can tempt you to fall into despair. Score one for the Dark Side if that happens.
I recommend caution about the situations you allow yourself in if you start to suspect someone is the returning evil for good sort. For example, he or she might accuse you of sexual assault if you don't give in to some demands, perhaps for money, perhaps something else. He may accuse you of smuggling drugs if you don't agree to get drugs for him. There are some people who detest goodness in others; and their intention is to corrupt them if they can or bring them down to destruction if they can't corrupt them. They can spin all kinds of stories; and maybe other people will believe them.
Why would anyone detest goodness in others? It makes them feel guilty; and every time they choose to detest good, they're hardening their hearts again in a vain attempt to sooth their conscience. Hell was made for (by) such people. They can be among others like themselves without feeling too guilty. It's ironic really that people can become so twisted that they prefer misery over being around goodness. Yet it is so, and I think you can observe it in some people. Another thing the conscience tells them is that they shouldn't be around good people. They prefer to be among bad people since they feel less guilty when they offend.
Evil often suspects good of wishing to torment it. It makes sense in a twisted sort of way since goodness in others can torment the conscience of evil.
Luke 8:28 When he saw Jesus, he cried out, and fell down before him, and with a loud voice said, What have I to do with thee, Jesus, thou Son of God most high? I beseech thee, torment me not.
Why would he imagine that Jesus would want to torment him? It really is strange, I think; but some people can get so far gone, they desire torment. Some like Pharaoh are even willing to commit suicide if they can't "win."
If someone returns evil for good, beware.