Just asking (6) : why would an ignorant sinner needs mercy?

On first look, it seems so unsuspecting straightforward.
A sinner of course needs mercy.

But thinking a bit more, why would an ignorant sinner needs mercy?
Just asking.

I suppose a good God will not need mercy to overlook an offence if committed by a mentally incapable person.
Equally an ignorant person.
Justice demands not exacting a sentence.
It shouldn’t call for mercy.

On the other hand then, if someone blatantly ignores the law, it is a different story.
Letting such a person go scot-free would definitely require grace and mercy!

 “. . . even though I used to blaspheme the name of Christ. In my insolence, I persecuted his people. But God had mercy on me because I did it in ignorance and unbelief. Oh, how generous and gracious our Lord was!” (1 Tim 1:13-14, NLT)

Couldn’t Paul have meant not “ignorant” but “ignoring”?
That certainly justifies his feeling of God’s grace and mercy!

Just asking.