Robot Monster(1953)

Moon monsters launch attack against Earth! How can science meet the menace of astral assassins? New Science Fiction Thrills!

Ro-Man, an alien robot who greatly resembles a gorilla in a diving helmet, is sent to earth to destroy all human life. Ro-Man falls in love with one of the last six remaining humans, and struggles to understand how his programming can instruct him to kill her while his heart demands that he can't.