As far as I can tell, neither the books or the film make this particular scene very clear. I will describe the scene from the film's retelling because that is currently fresh in my mind.
Katniss returns to the lightning tree after Johanna has unceremoniously ripped the implant out of her arm and then lead the Careers away from her. She finds Peeta has vanished and sees Beetee's body fly back from an explosion at the edge of the Arena. Beetee was holding a stick with a piece of metal at the end and tied up with his precious wire that leads back to the tree.
My intial assumption is that Beetee tried to somehow interfere with the forcefield using this stick. However, his character is clever enough to have previously gained a comprehensive understanding of force fields, as is demonstrated during the training sessions and his interest in hanging on to the long coil of wire and attaching it to the lightning tree. He also recently witnessed another tribute die due touching the forcefield whilst he was with Johanna and so must understand how dangerous it is to interact with the force field. He therefore must have known that (a) he would get harmed by touching the forcefield and (b) the force field would not be destroyed unless the stick was currently conducting lightning, which it was not. Any misunderstanding of the force field is betraying his character.
Was there something in the books that later on explained Beetee's point of view? Did I miss something out of the scene? Or are we simply restricted to the scene as Katniss witnessed it?