If God created everyone with a conscience, why have some people done such horrible things?

1 Timothy 1:19Cling to your faith in Christ, and keep your conscience clear. For some people have deliberately violated their consciences; as a result, their faith has been shipwrecked.

Sin is the act of going against your conscience. You know what you are doing is wrong because your conscience tells you so, but you do it anyway. If you consistently go against your conscience, you can train yourself to not hear it calling out to you, warning you of danger. You then become insensitive to sin. In a sense you have “tricked” yourself into thinking all is well when it really isn’t.