The D.A. (1971)

A Deputy District Attorney suspects that a nurse has been murdering her rich husbands and relatives by giving them unneeded insulin doses, but his superiors don't believe him.

D.A. Paul Ryan doesn't buy self-defense when a pharmacist fatally shoots an armed robber and brings the man up on manslaughter charges, for which he's convicted. However, when Ryan discovers the pharmacist's double life--he's been running a burglary ring out of the pharmacy, and the dead man may have been a member of said ring, he vacates the manslaughter conviction and sets out to nail him for murder instead.