This can’t have been easy for Saul. He became king, even though it wasn’t what he wanted, secured Israel’s borders and his own throne through a string of battles, only to be told he would eventually lose it all.
Kings have to meet a high standard in obedience; Saul didn’t. As Samuel said:
1 Samuel 15:26 Since you have rejected the Lord’s command, he has rejected you as king of Israel.
So Saul was king for now, but not a king for the long term. His sons wouldn’t succeed him. It was only a matter of time before God brought someone along to replace him. What a recipe for depression and paranoia!
1 Samuel 16:14 Now the Spirit of the Lord had left Saul, and the Lord sent a tormenting spirit that filled him with depression and fear.
You could argue until kingdom come about whether God did actually send the evil spirit, or whether it was just a turn of phrase. Either way, Saul’s life wasn’t happy after that moment.