Originally Posted by DontRunMeOver

Oojay's right in that the verbs criminalize and decriminalize are suggested to be opposites by the de- prefix.

But if criminalize means "to make more illegal".

Then decriminalize means "to make less illegal".

If you take something that is legal and make it more illegal, you are making it illegal. If you take something that is illegal and make it less illegal, it can still be illegal.

By more legal and less legal here, I refer to the severity of punishment meted out for committing the act. More illegal = more severe punishment.