The Hebrew here carries a scribal variant that changes how hard the second line reads.
Quick answer
Paying attention to how you live keeps you alive. The verse is about the preservation of a life, not about the destination of a soul.
Read both lines
The first line repeats a verb: he who keeps the command keeps his nephesh, his own life. Guarding and being guarded are the same word, which is the whole argument in miniature. The second line names the opposite man with a verb the NKJV footnotes as literally despises his ways, so he is not merely absent-minded, he holds his own conduct in contempt. This is the ordinary wisdom of the book, that a life handled carelessly tends to end early, and the "commandment" in view is the instruction this father has been giving his son since chapter one.
Why translations differ
The received Hebrew text preserves two readings of the last word. The written consonants give yumat, "shall be put to death," and the reading tradition gives yamut, "shall die." The NKJV follows the second, which is the standard choice, and the difference matters: the first implies an executioner and the second states an outcome. Nothing in either reading concerns final judgment. The catalog's settled position holds here as everywhere: nephesh in this book means a life rather than an immortal soul, demonstrated at 12:10 where it belongs to a farm animal, and no doctrine of anyone's eternal destiny may be built on a proverb in either direction.
Related: Proverbs 12:10 · Proverbs 13:13 · Proverbs 16:17 · Proverbs 4:4
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.