Based on testimony and evidence presented at the Reed trial:
He was not paying attention during his gun training.
He broke gun protocals throughout the filming. Including fireing a (blank loaded) gun after cut was called.
(The actual event) He pointed a “prop” gun at a real person and pulled the trigger. Even with a cleared gun, this is something that is not supposed to be done. Additionally, this was during a “blocking” session, so the camera was not even rolling. He was not supposed to be using the prop gun at all for this.
Even if the gun was loaded with blanks, this even would likely still have caused an injury (and possibly death, although likely not).
Baldwin will likely argue that Reed was supposed to know all of this and stopped him before the accident happened.