Actually the guy fully meant to fire his weapon... he just didn't know that his muzzleloader had anything in it other than the percussion cap. Which makes him unsafe with firearms. Dwain Haggard of Montana, A superintendent was showing a class of students his gun when he placed a cap on it and aimed it and pulled the trigger, expecting only a pop of the cap igniting. He and the class got a loud bang and a hole in the wall.
No one was injured and because he's not from a nanny state it looks like he'll just go from this with experience (Hopefully of learning to check, double check and even triple check to make sure a firearm is not loaded before dry firing, Or in a presentation such as his) and the shame of having made an honest, yet fully avoidable "Oops"
I'd like to know what was loaded in his gun, and if it was from him, a student playing a trick, or possibly his own children if he has any. Best bet is that the last time he used it he loaded it and never put on a cap to fire, maybe he was target shooting and got rained out, forgot it was loaded.
All in all there is no excuse for this accidental discharge. checking and obeying the four rules of gun safety.