Davis ordered Beauregard to reduce the fort by force if necessary before the resupply mission arrived. Beauregard followed those orders, Anderson would not surrender the Fort. Beauregard order his artillery to open fire. Will gladly listen to what your insight, supported by something more substantial that your opinion, into Jefferson Davis believed they were going to do.
He said if Beauregard believed the agent. (Robert Chew)
Beauregard obviously believed the agent. I also have little doubt that word was "flashed across the wires" as David Porter told Lincoln it would be.
This confirmation from Robert Chew is what triggered the bombardment. Prior to that they had told Anderson if he would but name a date to evacuate, they would wait while taking no action. They told Anderson that if he agreed to not fire on them if those Warships engaged, they would not fire on him. He refused.
A reasonable man would believe that when the President sends 8 Ships, at least half of them armed warships, it means he is going to use those ships. One does not ordinarily pull out a gun and threaten someone unless they mean to do something.
What were those warships going to do? What were the Confederates led to believe they were going to do?