Albert Barnes Commentary 1 Samuel 18:10

"And it came to pass on the morrow, that an evil spirit from God came mightily upon Saul, and he prophesied in the midst of the house: and David played with his hand, as he did day by day. And Saul had his spear in his hand;" — 1 Samuel 18:10 (ASV)

He prophesied - This, as the effect of the evil spirit coming upon him, is unique to Saul but is supported by what we read in 1 Kings 22:22. (Acts 19:15; 1 John 4:1–3). It is impossible to give the sense of “raving” to the word “prophesied,” as though a merely natural state of frenzy were intended. The “prophesying” here was as directly the effect of the coming of the evil spirit upon Saul as the “prophesying” in 1 Samuel 10:10 was the effect of the Spirit of God coming upon him. At the same time, it is quite true that “madness” and “prophesyings” were considered to be closely related (2 Kings 9:11).