YHWH's Response To Their Actions (1 Samuel 15:10-11).
YHWH's response was to reject Saul from being king on the grounds of sacrilege and high treason, that is, because by his actions he had rejected his Overlord's commands, and had committed sacrilege against what belonged wholly to YHWH.
‘ Then came the word of YHWH to Samuel, saying,'
The word of YHWH came to Samuel. He alone represented the true voice of YHWH. He was still very much YHWH's representative, with his authority still acknowledged by Saul.
“ It repents me that I have set up Saul to be king, for he is turned back from following me, and has not performed my commandments.” And Samuel was angry, and he cried to YHWH all night.'
YHWH declared to Samuel that He regretted setting up Saul as king because he had turned from following Him and had not obeyed His commandments. ‘Repent' is an anthropomorphism indicating what it looked like from a human point of view. It simply indicated that as a result of Saul's disobedience YHWH would now see him and act towards him differently. For the one thing above all others that He required in His ‘anointed one' (verse 1) was obedience.
To Samuel what he learned was devastating, for he recognised what it demonstrated, that Saul could no longer be trusted to do what YHWH required, even in the most serious of matters. His kingship had gone to his head. The result was that he was furious with Saul, and spent the night mourning because Israel's king, whom he had appointed, had been a total failure. And perhaps at the same time he was praying for God to show him what he should now do to prevent catastrophe for Israel.