I agree with everything you said but didn't really see it so clearly until you pointed it out. The phrases "risks everything" and "tyrannical forces of prohibition" are, as you said, generic and unnecessary. Taking on board your advise to not include the insect and prompted by patrockd's reply I offer this revision.
When the government start to kill the junkie population, a self-righteous policeman fights to save his drug using sister.
How does that sound? This has been really interesting for me. Thanks for taking the time to reply.