God, through his prophet, Samuel, commands that all the people (including INNOCENT BABIES AND NON-COMBATANTS) be slaughtered. When Saul fails to do so he is punished by God.
The usual apologetics for this is that Amalek, the nation, had harrassed and attacked the Israelites. I would understand if God commanded Saul to fight the soldiers of Amalek. I do NOT understand the need to slaughter the children. That is, technically speaking, murder.
And God is commanding his people (not long after giving the commandment to NOT murder) to murder people.
(Don't bother with excuses like "they had to kill the infants or they would grow up to be evil Amalekites and attack Israel... murder is murder even with an excuse.)