The 'evil spirit from the Lord' that afflicted Saul may simply mean a harmful/distressing spirit (Hebrew ra = bad/harmful, not necessarily morally evil), and God allowing adversarial spirits as discipline for rebellion is not itself immoral.

The Hebrew word ra can mean harmful rather than morally wicked. God using enemy forces — human or spiritual — to discipline rebels is a recurring biblical pattern (Nebuchadnezzar destroying the Temple as an example). God is not directly causing evil; he is sovereignly permitting and guiding it toward his ends.