A holy person is reminded of the evil in their own heart when they see others sin. Bernard tells of an old man who would weep whenever he saw someone sin. When asked why he grieved so deeply over another’s wrongdoing, the man replied, “He fell today, and I may fall tomorrow.”
The sins of others make a holy person reflect on their own sinful nature. Other people’s wrongdoings serve as mirrors, helping them see the many seeds of sin in their own heart. This awareness leads to humility.
A holy person knows that one of the best ways to keep themselves pure is by grieving over the sins of others. Someone who makes it a practice to mourn for the sins of others is less likely to be stained by them.
A holy heart sees the sins of others as chains that bind them, which brings sorrow. How could tears not fall from a Christian’s eyes when they witness crowds bound by their iniquities, heading toward Hell? Who can look at a lost sinner, a prisoner to the powers of darkness, and not be moved to sorrow?
If godly people mourn for the wickedness of others, what can we say about those who find pleasure in it? Those who laugh and rejoice at the sins of others, making light of their wrongdoings, are more like monsters than men! No one is more closely related to Satan than these, for Satan himself delights most when people sin most. To praise or enjoy those who find pleasure in sin is the height of ungodliness!
-Thomas Brooks (updated for readability)
The Fight of Faith 