Contradiction with Exodus 20:13
While 2 Kings 25:21 describes execution, Exodus 20:13 commands "Thou shalt not kill," opposing such actions.
Exodus 20:13: Thou shalt not kill.
Contradiction with Matthew 5:44
2 Kings 25:21 involves violence and hostility, whereas Matthew 5:44 teaches to "love your enemies" and promote peace.
Matthew 5:44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Contradiction with Romans 12:19
Romans 12:19 advises against vengeance, saying, "Vengeance is mine; I will repay, saith the Lord," contrasting with the retributive justice in 2 Kings 25:21.
Romans 12:19: Dearly beloved, avenge not yourselves, but [rather] give place unto wrath: for it is written, Vengeance [is] mine; I will repay, saith the Lord.