describe the moral decline of saul?

If we're talking about the Saul in the Bible, his moral decline, as you say seems to be driven as much by the pressure applied by Samuel as anything from inside Saul.

He never wanted to be king. He wasn't trained for the job in any way. He tried to avoid it. He tried to do his best on the job, but it was never good enough for Samuel. Samuel was resentful of the fact that the people wanted a king instead of being ruled by him as Judge, and thereafter by his sons, and he did everything in his power to undercut, and destroy Saul.

Remember, the history of this period was written by David's people, and they made David, who wasn't by any standard a very good man, the big hero. "Saul has slain his thousands, but David his ten thousands," the people cheered. David had killed Goliath. That's all.

Saul's decline wasn't moral, but psychological, being driven slowly crazy by unfair pressure from Samuel and David's treason, and from responsibilities that under the best of circumstances were hard for anyone.