Swamp Thing
(1982)

Ray Wise is deep in the swamp hard at work on a botany project to feed the world. Louis Jourdan is the villain who wants it for himself.

During an armed confrontation, the lab blows up and Ray Wise turns into SWAMP THING (Dick Durock).

Jourdan figures out that Swamp Thing was Ray Wise and takes "the secret formula" to achieve "greatness", but turns into a monster that Swamp Thing defeats. Adrienne Barbeau tries to convince Swamp Thing to go back to the lab but he sadly returns to the swamp.

Thanks Don P!