Again, depending on just how the theme of eugenics is to be  
developed, there are a range of classic films.  If it is taken as not  
just "purifying" but "improving" the race (or "human stock"), then  
one theme that fits in is robots.  That is, in Capek's "RUR" (1924)  
artificial workers are created so humans don't have to do this kind  
of labor (and since they are machines, eliminating all sorts of  
social problems--poverty, drunkeness, unwanted children [and the need  
for birth control], etc.).  There is probably a strong air of it in  
all robot/genetic engineering films/stories--manual labor is brutish,  
and the humans doing this are kinds of/closer to brutes/mere  
animals.  Getting rid of them improves the race, and human life.   
This, along with some notion of "Brave New Worldish" strata of social  
organization, may well underlie such films as "Metropolis" [c.1926]  
and probably "Frankenstein".  Charles Laughton's Moreau in "The  
Island of Lost Souls" (1933--one of the first Wells's adaptations,  
and still probably the best) is a chilling figure.

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