Sen the Professional 
I wouldn't pull the lever. (Or, in the variant, push the person off the bridge.)
Because that's murder.
If I don't pull the lever (or push them), 5 people die.
But I'm not responsible for their deaths. There was no legal action I could have taken.
At most I'll be giving a statement to police (assuming I don't book it), not getting shoved in the back of a squad car.