Batman and Superman are two formidable fellows, and if both of them are going to be present in the same feature film (in addition to the presence of a third superhero, Wonder Woman), then one might argue that such a film would require the presence of a monumentally robust super-villain that could present a proper physical challenge to the likes of someone conceivably invincible as Superman.
Regarding the presence of Doomsday in Batman v Superman, director Zack Snyder said:
To go up against heroes like Batman and Superman, we wanted to introduce one of the DC Universe’s most iconic and powerful villains. He is essentially an unstoppable force. He poses a real threat to the world within the film.
Considering Snyder’s statement and the logic of including Doomsday in the film, one might concur that the inclusion of a malevolent force that is equal to (or arguably greater than) Superman should tip the balance of Good and Evil in such a way that naturally causes the Justice League to unite, hence the subtitle of “Dawn of Justice;” however, let’s just watch the film to find out.
Snyder also spoke on Lex Luthor’s interest in meta-human power, a concept that relates directly to the existence of the likes of Doomsday, Superman, and Wonder Woman.
You learn pretty early on that Lex is interested in meta-human existence, and that there might be some relationship between that investigation and the appearance of Diana Prince [Wonder Woman].
Who’s excited to see Doomsday?