Hell yeah he kicked him into the engine intentionally. Thats what was so great.
And about not "killing" and unarmed man. He said that to the Doctor. He meant he wouldn't betray him. He won't shoot you in the back. He'll face you like a man. He did that to Niska's goon and then to Jane. He could have just as easily spaced Jane but he gave him a shot at explaining himself.
He gave the goon a shot to. All he had to do was tell Niska, deals off no bad feeling. Goon didn't want, oh well, *kick*, next. Firefly rocks.