Then, you're still applying responsibility to a guy who may have been temporarily insane driven by an irresistible impulse.
He is responsible if he shot the guy... who else would be responsible for the man's death if the Dad got a gun and shot the guy???
The dad would have been the one who decided to end his life, whether he did it while under distress or insanity.
Did the driver decide to end an 11 and 12 year old's life when he got behind the wheel? Maybe, maybe not... we will never know.
ETA: from the report in the OP, they don't know if the driver was drunk, they don't know that the dad shot the driver.
But if the OP assumptions are correct, we must consider whether the driver intentionally killed the kids, and whether the dad intentionally killed the driver.
This post was edited on 2/13 at 2:47 am