If you can't handle the distinction between murder and killing, then, again, just stop.
How do you define the difference between murder and killing?
There are many possible differentiators, here are a couple that come to mind:
Murder is killing for no reason.
Murder is killing for no *good* reason.
I'm just curious as to how you might distinguish between the two without resorting to something like "I know it when I see it".
It's a poor workman who blames his tools.