Hi,
Theft isn't the issue here. The heroin is already an illegal substance and Mick would not be able to pursue a matter in the courts. He would be saying, "Hey, he stole my heroine', which is also, "Hey, he stole an already illegal (and accordingly illegally acquired) from me!" The courts wouldn't be very sympathetic to this situation...
I don't want to give to much of an answer but think of the replacement of a substance which he KNOWS to be lethal...
Hope I've helped a little...