Sin is rebelling against God's law:
1 John 3:4
"Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness."
Romans 5:12
"Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned"
Sin is a problem we all have:
Romans 3:23-24
"for all have sinned and fall short of the glory of God, [24] and are justified by his grace as a gift, through the redemption that is in Christ Jesus,"
Romans 3:9-10
"What then? Are we Jews any better off? No, not at all. For we have already charged that all, both Jews and Greeks, are under sin, as it is written: “None is righteous, no, not one;"
Sin is following our own ways and leads to death:
Romans 6:23
"For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord."
Ephesians 2:1-2
"And you were dead in the trespasses and sins [2] in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience—"