Why should a man who stole a medication should be prosecuted?

There is an old expression among attorneys that "hard
cases make bad law." The situation you describe is one of those "hard cases."
Nonetheless, it is necessary that laws be enforced uniformly. If exceptions are made on
the basis of circumstances, then there would be a never ending series of convincing
stories to justify one's actions. Theft is theft, pure and simple. It is possible that
if there were extenuating circumstances, he might be treated with a degree of leniency.
Still, his theft was most likely the result of poor judgment rather than need. Our legal
system is far from perfect; there are numerous situations where the punishment seems
disproportionate to the crime; yet imperfect humans must deal with the imperfect
institutions they have created as best they can. There is no suitable
alternative.