“Then David said to Saul, ‘I cannot go with these, for I have not tested them.’ So David put them off.”
There was nothing wrong with Saul’s helmet and coat of mail; they worked just fine for Saul.
There was nothing wrong with Saul’s intentions; at the time he thought well of David. He had no desire to sabotage David’s efforts and every reason to hope he might succeed against Goliath.
And there was definitely nothing wrong with David; Saul’s armor just didn’t suit him.
Sometimes other people’s methods don’t work for us.