When we think someone’s wrong it’s tempting to engage others in deciding they deserve to be punished. It’s a claim to know the heart and motive, the background and circumstances of another’s choices.
But God says we’re not to speak evil of each other, for this is speaking evil of the Law and judging the Law (Ja 4:11); it’s putting ourselves above the Law, usurping the place of the Lawgiver. (Ja 4:12)
It’s one thing to identify an act as sinful, contrary to God’s Law (1Jn 3:4), but declaring someone to be evil, that they’re willfully alienating themselves from God and at enmity with Him, is another matter. This is actually deciding the law itself is contrary to God, since it tells us to love our neighbor as ourselves (Le 19:18), which we aren’t as we take pleasure in condemnation.
If we shouldn’t speak so about others, we shouldn’t be thinking this way either. Rather, in humility, we should be giving them the benefit of the doubt, esteeming others better than ourselves. (Php 2:3)
Only God can judge the heart; we must leave this work to Him. (Ps 119:84) He doesn’t need our help. God is just, and perfectly so; He’ll do just fine, all by Himself.