BK I think what you are trying to say is don't go easy on them because they were on drugs, not don't blame the drugs. There is a pretty good chance that had they not been fried they wouldn't have done what they did, so the drugs are to blame, at least partly. But, I agree with you 100% that they should still be held fully responsible for their actions and should not be let off lightly because they were on drugs.
I know there are people who use drugs regularly and are otherwise normal law abiding citizens. The problem isn't so much caused by the drugs themselves, but the desperation of an addiction that consumes your life and distorts your perspective.