Jamieson, Fausset and Brown on Ezekiel 18:21-24

21-24. Two last cases, showing the equity of God: (1) The penitent sinner is dealt with according to his new obedience, not according to his former sins. (2) The righteous man who turns from righteousness to sin shall be punished for the latter, and his former righteousness will be of no avail to him.

he shall surely live--Despair drives men into hardened recklessness; God therefore allures men to repentance by holding out hope [Calvin].

To threats the stubborn sinner oft is hard,

Wrapt in his crimes, against the storm prepared,

But when the milder beams of mercy play,

He melts, and throws the cumbrous cloak away.

Hitherto the cases had been of a change from bad to good, or vice versa, in one generation compared with another. Here it is such a change in one and the same individual. This, as practically affecting the persons here addressed, is properly put last. So far from God laying on men the penalty of others' sins, He will not even punish them for their own, if they turn from sin to righteousness; but if they turn from righteousness to sin, they must expect in justice that their former goodness will not atone for subsequent sin (He 10:38, 39; 2Pe 2:20-22). The exile in Babylon gave a season for repentance of those sins which would have brought death on the perpetrator in Judea while the law could be enforced; so it prepared the way for the Gospel [Grotius].