Sin is destructive behavior that brings intentional harm to yourself, others, society and those representing others or God.
1 John 3 4 Everyone who sins breaks the law; in fact, sin is lawlessness. 5 But you know that he appeared so that he might take away our sins. And in him is no sin. 6 No one who lives in him keeps on sinning. No one who continues to sin has either seen him or known him.
Mark 12 28 One of the teachers of the law came and heard them debating. Noticing that Jesus had given them a good answer, he asked him, “Of all the commandments, which is the most important?”
29 “The most important one,” answered Jesus, “is this: ‘Hear, O Israel: The Lord our God, the Lord is one.[e] 30 Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’[f] 31 The second is this: ‘Love your neighbor as yourself.’[g] There is no commandment greater than these.”
The sum of the law is love - of God, yourself and your neighbours.
If sin is lawlessness, then sin is the opposite of that - hate towards God, yourself and others.
Therefore, all acts of "sin" as postulated by the society (e.g. immoral behaviour, adultery, murder etc), according to the Word of God are merely the symptoms of sin.
Therefore, real sin or the root sin is the very fact that we are not living according to God's ways, but "missing the mark" and going on our own ways.