What's the meaning of 終末 in 終末の微笑?
I know 微笑 means smile, but would 終末の translate to "The final", "The last" etc. Would you be able to say 最後 in 最後の微笑 and it would have the same meaning?
In general, 終末 is a "heavy" word that usually refers to the end of the world, the apocalypse, the termination of someone's life, the finale of a saga, and so on. Judging from the context you provided, this 終末 refers to the termination of a girl's life. It's simply "The Last Smile" in English anyway, but 最後の微笑 may make the title look lighter.
("It's the title of a song and these are the lyrics" is a huge context! If you want a good answer that is any better than a dictionary definition, please always add how the word or phrase is used.)