My answer...my gut-instinct, this is the only way for me to behave answer, is: my actions here have nothing to do with Johnny. Murder is a sin, and murder has, as Kaiya said, emotional consequences. So for Larry's sake--not Johnny's--I should stop Larry from killing Johnny. I believe we are obligated to protect our friends from things that they may do, in the heat of the moment, that will later come back to haunt them. At least on the scale of murder and such.
It may help you to understand my answer if I admit that my second reaction on reading this was to think of my favorite werewolf and a similar problem he was part of. If "Larry" was temporarily insane or somehow couldn't help his actions...and there's no way to know, just from the scenario as given, if that is the case.
Anyway, go comment on Ethics Scenario #1a now.