Learn Russian: В конце фильма убийцу нашли. - At the end of the film the killer was found.

Russian often uses a subjectless plural to express what English does with the passive voice. By using the plural past tense нашли (they found) without a subject like они (they), the sentence focuses on the action rather than who did it, meaning "someone found the killer" or "the killer was found."

Why is убийцу in the accusative case here instead of the nominative?

Because grammatically, this is an active sentence with a hidden subject. The unspecified "they" is doing the finding, making the killer the direct object receiving the action (they found whom? the killer). As a direct object, убийца takes the accusative ending -у.

The dictionary form убийца ends in -а. Does that mean it is a feminine noun?

Not exactly. Убийца is a "common gender" noun. Its grammatical gender matches the actual gender of the person it refers to (so if the killer is male, you use masculine adjectives). However, regardless of the killer's gender, the word always declines using feminine-style case endings. That is why it changes to убийцу in the accusative case here.

Why do we use the prepositional case for в конце but the genitive case for фильма?

В конце is in the prepositional case because it points to a location in time (answering "when?" or "at what point?"). Фильма is in the genitive case because it links two nouns to show belonging or relation, functioning just like the English word "of" (the end of the film).