What had Adam done? He was "the one who sinned" (Ro 5:16). What is sin? It is a "transgression" (:15) or "offense" (:15 KJV). It is also a "trespass" (:15 RSV). You've seen signs saying "NO TRESPASSING." "God commanded . . . you shall not eat" (Ge 2:16-17). But Adam violated the command. "Through the one man's disobedience the many were made sinners" (Ro 5:19). The result was "through one man sin entered into the world, and death through sin" (:12). But others "had not sinned in the likeness of the offense of Adam" (:14). Then "death spread to all men" (:12) because "in Adam all die" (1Co 15:22). Sin is the culprit because "the sting of death is sin" (1Co 15:56). "The wages of sin is death" (Ro 6:23). Therefore "sin reigned in death" (5:21) and "death reigned" (:17). "Death works in us" (2Co 4:12). One might respond that everyone has to die naturally anyway. But you should consider "him who had the power of death, that is, the devil" (Heb 2:14).
"By a man came death" (1Co 15:21) and "you will all likewise perish" (Lk 13:3). Death is firstly interpreted as physical death. "Enoch was taken up so that he would not see death" (Heb 11:5). "It is appointed for men to die once" (9:27). Sin is the reason for the problem. You are "dead in your trespasses and sins" (Eph 2:1). Consequently you "shall be condemned" (Mk 16:16) and therefore must be "saved from wrath" (Ro 5:9). Sin contributes to the inevitable. It is "cause and effect" because it's a "law of sin and of death" (8:2). "The outcome of those things is death" (6:21) since there "is a sin leading to death" (1Jn 5:16). It is said we live in the flesh. However, "if you are living according to the flesh, you must die" (Ro 8:13). "The mind set on the flesh is death" (8:6). "With my flesh [I serve] the law of sin" (7:25) "resulting in death" (6:16). "While we were in the flesh, the sinful passions . . . were at work in the members of our body to bear fruit for death: (7:5).
How would you answer a person who asked, "Why would I have to repent if I didn't sin like Adam?" (Ro 5:14). Adam sinned because he broke God's commandment (Ge 2:17). He disobeyed (even rebelled) because he was not deceived (1Ti 2:14). Perhaps he ate the fruit Eve gave him because he just wanted to keep her happy. Maybe he rationalized that death would be only a temporary disciplinary action. It's possible he considered knowing more and becoming wiser would be worth it. "Through the one man's disobedience the many were made sinners" (Ro 5:19). It's as though we, as descendants, have inherited the "original sin." Since "by a man came death" (1Co 5:21), through solidarity with Adam, "death spread to all men, because all sinned" (Ro 5:12). "Like Adam they have transgressed the covenant" (Hosea 6:7). Our guilt is imputed in a legal way as if a contract has been violated and a judgment has taken place because the ground has been cursed (Ge 3:17-18). This is the Adamic Covenant where Adam, as the representative of the human race, causes God to introduce physical death saying, "'You [will] return to the ground, because from it you were taken'" (:19). Others may say that Adam's nature became corrupted by the sin and that we inherit the corruption and sin automatically (Ro 5:19). Calvinists believe that we are incapable of being spiritually saved except for God's grace. So if you repent for these reasons you acknowledge that "through the obedience of the One the many will be made righteous" (:19).