1 Samuel 16:14 refers to an evil spirit of the Lord. What is this spirit?

The Bible mentions several occasions in which “a harmful spirit from the Lord tormented” Saul (1 Sam. 16:14; 18:10; 19:9, ESV).  Some understandably view this as an Old Testament example of demon possession, but this view is mistaken when one closely compares Saul to the New Testament examples of evil spirits.  In the New Testament, miraculous power was required to cast out evil spirits from people (Matt. 17:14-18; Lk. 11:14; Mk. 1:32-34; etc.).  Yet in Saul’s case the Bible says that David was able to refresh Saul by playing his lyre, thus causing the harmful spirit to depart from him (1 Sam. 16:23).  In the New Testament, casting out demons was considered to be a miraculous act.  But in Saul’s case, all he needed was soothing music. 

This shows that Saul was probably borderline insane and had severe mental problems rather than actually being demon possessed.  Thus, “the harmful spirit from the Lord” was likely mental problems.