Translation:The conquest of evil is the theme of many movies.
"Venko" translates as conquest and victory, so is this sentence supposed to be saying that many movies are based on evil being victorious? Or is this Esperanto construction - "venko de ......... " supposed to translate as what we would say in English as "victory over ........?"
The clearer alternatives are "venko super malbono" / "venko fare de malbono."
PMEG waffles on this point, saying only that "de" may indicate either agent or patient.
PAG (S135-136) goes into more detail, (it really is la pli plena of the two). The conclusion is pretty complex with five branches:
1 - If the base verb is typically used with human subject and nonhuman object, de gives you the normal relationship in either case. skribo de la reguloj / skribo de Adamo
2 - If the base verb shows a relationship between people (alternsubjekta), then de indicates the patient (person affected). amuzo de gasto / amuzo per muziko / amuzo fare de kantisto
3 - If the base verb shows an attitude (sinteno) de is the subject (person having that attitude). If the attitude is toward an object, de may also be used there. amo de Adamo al/de ananasoj
4 - If the base verb shows mental activity (spiritfunkcia), the best practice is the same as 3, but use pri instead of al - havas revon pri Duo
5 - If the object may be a person or thing, de should most likely point to the object.
Venko belongs to the 5th category, so much like in English, this "conquest of evil" most naturally means that evil is being conquered. But "conquest of evil over kindness" would very clearly mean the opposite.
Words of the 3rd and 4th kind are different from English. "Love of/for children" -> "amo al infanoj," "thoughts of travel" -> "penso pri vojaĵo." The other rules pretty much match how things are done in English.