Also, what would happen if you pushed a guy off a cliff? The ground would kill the person, but you would of killed the person because you caused the person to fall off the cliff, resulting in his/her death. If you push somebody into the path of a train on purpose, the train conductor would not be a murderer. Nor would the train. You would be.
If you are still unsure, try pushing somebody into a busy highway so a car will hit them. Try explaining to the police the driver killed them.