The Alligator People
(1959)

 Joyce and Paul Webster are newlyweds waiting on a train to leave on their honeymoon.  Paul receives an urgent telegram, and gets off the train to make a call.  He does not return.  After weeks of investigation, Joyce finds that on his college application he listed an address of The Cypresses Plantation.  She goes there to search for him.  The Cypresses is a creepy isolated plantation in the middle of the swamps.  The lady of the house, Lavinia, is hostile, and says she has never heard of Paul Webster.  Since there is no other train, Joyce has to stay the night.

Years before, Paul had a serious accident.  The local doctor did an experimental treatment to regrow limbs by injecting alligator hormones.  Now, Paul and the other patients are turning into "alligator men".  The doctor has a plan to treat him with gamma radiation to eliminate the reptile traits.

Joyce is assaulted by the creepy caretaker (Lon Chaney Jr).  Paul rescues her and gives the caretaker a beating.  They reveal the truth about Paul to Joyce. During the gamma radiation treatment, the caretaker breaks in and attacks Joyce and the doctor.  Paul is left in the machine way too long and turns into a reptile, with an alligator head.  The caretaker falls into the gamma ray machine and dies.  Paul runs into the swamp.  Paul sinks in quicksand and dies.  

Thanks Joseph C!