Why Adonijah was Killed -- No Wilfull Return to Sin

17And he [Adonijah] said, Speak, I pray thee [to Bath-Sheba], unto Solomon the king, (for he will not say thee nay,) that he give me Abishag the Shunammite to wife. 18And Bathsheba said, Well; I will speak for thee unto the king." (1 Kings 2: 17-18)

Adonijah's name means "Jah is my Lord"

When you know that "The LORD" is your Lord, there is no going back to a life of sin:

"What, shall we sin, that grace may yet more abound? God forbid." (Romans 6: 1)

and

"Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord." (Romans 6: 11)

When you believe on Jesus, that He is the propitiation of your sins (and He is the propitiation of the sins of the entire world, too! -- 1 John 2: 2) then you are saved (Romans 10: 9), and you are taken from dead in your trespasses to alive and seated in heavenly places in Christ Jesus (Ephesians 2: 1-2, 5-6)

We receive Christ's own righteousness (2 Corinthians 5: 21). In fact, we become sons of God in Christ (1 John 3: 1), a new creature (Galatians 5: 6)

So, we are dead to sin (Colossians 3: 1-4), and thus called not to respond to our flesh, but rather walk in the Spirit (Galatians 5: 16)

Yet Adonijah willfully asks to marry Abishag, whose name means "Source of Error/Wandering".

We are called to rest in Christ, not work under the law, or chase after grievous idols and false gods.

When Solomon, a type of our Lord Jesus, heard that Adonijah wanted to marry Abishag, he replied:

"Then king Solomon sware by the LORD, saying, God do so to me, and more also, if Adonijah have not spoken this word against his own life. 24Now therefore, as the LORD liveth, which hath established me, and set me on the throne of David my father, and who hath made me an house, as he promised, Adonijah shall be put to death this day. 25And king Solomon sent by the hand of Benaiah the son of Jehoiada; and he fell upon him that he died." (1 Kings 2: 23-25)

There can be no god to rival the Prince of Peace (Isaiah 9: 6). For the believer who has declared: "Jesus is my Lord", there is no more wandering, without death being the result:

"The labour of the righteous tendeth to life: the fruit of the wicked to sin." (Proverbs 10: 16)

More importantly:

"For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord." (Romans 6: 23)

Instead of our labor of righteousness, we bring every thought into captivity to the obedience of Christ (2 Corinthians 10: 5), for by His work, we are made the righteousness of God in Christ (2 Corinthians 5: 21)

Why Adonijah was killed? To fulfill the type of a man who has gone from dead in his trespasses to alive in Christ, there is not willful return to a life of wandering in sin and error, for we are to reckon ourselves dead to sin (Romans 6: 11)