What? He was fine. He was holding off the lightning and simply trying to justify to himself and to Anakin his decision to kill Sidious. The only thing holding him back was his moral convictions and his discussion with Anakin.
He was literally in the process of dealing the killing blow when Anakin jumped in and cut his hand off and knocked his saber away.
"don't listen to him anakin.. aaahhhhh!!!.."
Sidious ~ "I'm weak.. ahh..."
Few seconds later..
Sidious ~ "UNLIMITED POWER!!!"