That is why we have to be concerned with the ethics of our actions. It does not change. It underlies moral reality. Both objectively and coming down to the personal level, Right is always right, and Wrong is always wrong Good is always good, and Evil is always evil. Though other things can and do change, Good cannot become evil, nor evils become good over time.
Christian parents help to form their child's conscience when they teach right and wrong.
Listening to the 10 commandments, the teaching of the New Testament and the teaching of the Church is necessary to maintain a good conscience as these are revealed natural and Divine laws and truths. When you go against your conscience because of desire and temptation, you naturally feel guilty. In order to in again, you begin to find reasons to justify your actions. Repeated sin kills the conscience's discernment of good and evil. If in doubt, we must get more information from an authoritative source, come as close as we can to the Truth of the matter, and the objective and universal Truth must override personal subjective inclinations and opinions.