I do not think a sin is a sin because we think it is.
I believe it is a sin because God said it is.
If a person wants to argue the point with God at the Judgement, I suppose they could, but I am fairly certain they would loose.
But on a the real subject, what abput an improperly formed conscience? I suppose it would depend on why a person’s conscience is not in line with Church teachings.
Is it because of mental illness? Are they simply not very bright? In those cases, it is probably not the fault of the individual because they are impaired.
Could it be deception, that is was the person deceived really believes somethng is okay? This is far more rare that many people will admit. But if they have been lied to and they believe the lies, I think culpability is lessened, but certainly not eliminated. Adter all, did they try to find the truth?
Is it because they do not want to know? Willful ignorance is not a good thing. It is a form of denying the truth.
Maybe they are just lying to themselves. Self-deception is a very popular pastime among humans, few are completely immune. Myself in particular!

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Or perhaps they are looking for a loophole, a opportunity to justify a wrong they know they have done. This is either relativism or legalism. Neither one is particularly useful in developing a good conscience.
(By the way, my definition of a “good conscience” is one that is in line with Church teachings. But more than this, as the person learns more about their faith, their conscience is adjusted. They are not perfect, but at least they know when they have done something wrong.)