The murderous impulse of the killer.
"Trace it back .... to the first volitional action that would have changed the outcome.
I have no idea when the killer embraced his murderous character.
The kid's power of choice trumps "the murderous impulse of the killer" bacause he could have pre-emptively made a choice that would not have resulted in him crossing paths with the killer.
THAT choice, the choice to not be with a bunch of people throwing eggs at cars, is the first choice that would have directly changed the outcome.
Are you saying he did not have that choice?
Do you choose to walk through bad parts of town?
If not why not, could it be to avoid possible consequences over which you have no control?