If you harm someone, whether it be intentional or through neglect and dereliction of responsibility..you should be severely prosecuted and punished. No reason to increase OR lessen the sentencing simply due to the results of a blood test. Getting retarded high on weed and driving a forklift through a wall and crushing the man who was on the other side of the wall would fit under dereliction of responsibility, for example.
Being an incompetent driver and causing a wreck that kills someone by complete accident wouldn't count, unless you were getting high, or drunk. You should still suffer consequences, but not as severe as if you were drunk, or doing it on purpose.