I think it is important to consider what you mean by accountable, and at what point in the life of the sinner that accounting takes place.
From one standpoint, all have sinned. All sin separates the sinner from God. The wages of sin is death. These wages, which is a debt, has been paid for in the blood of Christ. All that is required is that the sinner acknowledges his need for divine redemption, that Christ provided that redemption, and accepts that payment.
From this standpoint, there is no small sin, only large sin, since it separates us from the Father. We are all sinners. We are each accountable for our sins. We are each unable to pay the price. Those that acknowledge this and accept Christ’s payment receive God’s forgiveness, making them spiritually justified, but this is not really non accountability.
From another standpoint, some sins do appreciable harm to ourselves and to others. This is a more immediately practical viewpoint, but that is only our viewpoint. God’s viewpoint is the same as the previous all have sinned, no small sin standpoint.
As for the effects of a brain that does not work well, we must remember that all sin is fundamentally wanting what seems best to our self regardless of others, or of God. It is rebellious in nature, even if it appears to be good from someone’s viewpoint. If the brain does not work well enough to form a rebellion, it is no more sinful than a mindless disease.
Being willfully out of one’s mind with drink, or drug misuse is rebellious in itself. Overcoming addiction can be one of the hardest things of all. Drink can be a big sin because of the difficulties one has in overcoming it, not because the cleansing power of Christ’s blood is somehow ineffective. It is temporally a great sin. But if one has truly acknowledged and accepted Christ’s sacrifice, the effects are only in this life. The saved sinner who cannot conquer his sin may not be growing in Christ, but the Lord is faithful, even when we are too spiritually weak. This is a defeated or carnal Christian.
An accounting of the defeated Christian during his life, is not to his credit in this life, and he will not grow in Christ until he accepts Christs help in overcoming his own self.
He can still enter the Kingdom as a spiritually immature child of God.