Perhaps they were Never Born Again

A truly born again believer is guilty in his heart about sin until he can no longer stand the weight of it and confesses it desiring to be cleansed by God.

1 John 3:9-10
"9 Anyone born of God refuses to PRACTICE sin, because God’s seed abides in him; he cannot go on sinning, because he has been born of God. 10 By this the children of God and the children of the devil can be distinguished: Anyone who does not practice righteousness is not of God, nor is anyone who does not love his brother.…"

The operative word in this passage is "practice", every believer sins, but they do not love sin, they are ashamed when they do it and eventually will confess and leave it.

When someone has sinned, refuses to confess and gets angry when told they have sinned, then it is possible and likely that they were never born again in the first place.

Matthew 7:21-23
"21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven.22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ 23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’