Contradiction with Deuteronomy 20:16-17
While 1 Samuel 15:4 involves Saul preparing to attack the Amalekites, Deuteronomy 20:16-17 commands the complete destruction of certain peoples, including Amalekites, showing the utter annihilation that contrasts Saul's initial assembly.
Deuteronomy 20:16-17: But of the cities of these people, which the LORD thy God doth give thee [for] an inheritance, thou shalt save alive nothing that breatheth:
Contradiction with Matthew 5:44
1 Samuel 15:4 describes an impending battle with the Amalekites, whereas Matthew 5:44 teaches to love your enemies and pray for those who persecute you, presenting a contradiction in approach towards enemies.
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 Exodus 20:13
This verse states "Thou shalt not kill," which contradicts the impending military action and killing depicted in 1 Samuel 15:4.
Exodus 20:13: Thou shalt not kill.
Contradiction with Romans 12:19
1 Samuel 15:4 describes preparation for vengeance against the Amalekites, while Romans 12:19 advises against taking vengeance, urging to leave wrath to God.
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.