John Gill Commentary 1 Samuel 28:18

"Because thou obeyedst not the voice of Jehovah, and didst not execute his fierce wrath upon Amalek, therefore hath Jehovah done this thing unto thee this day." — 1 Samuel 28:18 (ASV)

Because you did not obey the voice of the Lord
At Gilgal, as Abarbinel thinks; which is the first thing for which the kingdom was threatened to be taken from him, (1 Samuel 13:13 1 Samuel 13:14) ;

nor execute his fierce wrath upon Amalek :
in sparing Agag, and the best of the cattle, (1 Samuel 15:9 1 Samuel 15:28) ;

therefore has the Lord done this thing to you this day ;
forsaken him, rent his kingdom from him, and would deliver him into the hands of the Philistines, as follows: Had he been the true Samuel, he would have told him of some other sins of his, provoking to the Lord; as his slaughter of the priests at Nob, his cruel persecution of David, and especially of his asking counsel of one who had a familiar spirit at this time. About which not a word is said, and yet this was the very transgression for which Saul died, (1 Chronicles 10:13 1 Chronicles 10:14) .