We see an 'evil spirit' here, but it's described as 'from the LORD.' How can a good God send something evil?
This is one of those challenging verses where we have to hold two truths at once.
God's Sovereignty
God is ultimately in control of everything. Even the 'harmful spirit' comes to Saul because God permits it. This doesn't mean God likes evil or causes it in the way a human does. Instead, it shows His absolute power over all spiritual forces, good and evil.
The Spirit of Evil
This 'harmful spirit' (sometimes translated 'evil spirit') is distinctly different from the 'Spirit of the LORD' that empowers prophets and kings. This spirit brings confusion, rage, and a desire to destroy. It's acting against God's will, but God, in His sovereignty, allows it to come upon Saul as a consequence of Saul's rebellion and disobedience.