I'm going to have to strongly disagree. There are currently millions of non-violent offenders in prison simply for their connection to marijuana. That's a lot of people you're going to need to "replace" to fill up the prisons once again.
When you get high, you don't just go on a crime spree, lol. We're not talking Bath Salts or LSD or something that severely changes your perception of the world and ability to think rationally. You are the exact same person that you were before smoking the drug (specifically when we're talking marijuana); in fact, you're likely to be a bit more paranoid and cautious and much less likelier to commit a crime in my view.
No crimes, no problem, is the idea.
If everything is legalized, what would it matter if someone took a non-toxic drug in careful circumstances where they wouldn't kill or die or hurt someone?
But when they take bad drugs like meth or PCP or "bath salts" and, you know, eat someone's face off, that would send them to prison or they'd die soon. And everyone else would be better off.
I figure it might be quite a circus for 20 years, but then all the people who do that sort of thing would be dead or marginal and the rest would be smart enough to stay away from bad drugs.
I would pair this freedom with educational campaigns. Now there is not much education against drugs, not as much as there could be, because drugs are illegal so it's a bad idea to publicly assume everyone can get them anyway, though of course that is probably true to some extent. It could be like the anti-smoking campaigns. Assume people are freely doing this and hit it hard as incredibly stupid and unhealthy and generally bad. That would help young people and confused people, maybe.
The difference is, smoking doesn't make someone high. Drug addicts will chase that high to their deaths, and the deaths of anyone else too. The way to deal with those who feel good and get high, then eat someone's face off is really to allow more concealed weapons. Strengthen laws supporting self defense AND defense of others. There's no reason why these killers should go to prison. Drop them where they stand, save a lot of money.