The Mind Snatchers1972

Private James Reese (Christopher Walken), a misfit G.I. stationed in Western Europe, has landed in the stockade more than once thanks to his violent temper and even his girlfriend asks him to seek psychiatric help. Unexpectedly, Reese is driven to a secluded country mansion, a hospital-like facility with few patients or staff members. Without explanation he's assigned to a room with Miles (Ronny Cox), a young American suffering from terminal cancer and bouts of aggressive, violent behavior. Slowly Reese uncovers the horrifying truth involving medical experimentation, with humans used as guinea pigs in brain control research.