The ending of Underworld (2003) comes down to three moves: Selene kills Viktor, Kraven has already killed Lucian, and Marcus awakens as a hybrid. The explanation below contains full spoilers and is my own reading of the film.
Why does Selene kill Viktor? She learns that he murdered her family and lied to her for years, so the authority she served as a Death Dealer was built on a falsehood. In my opinion, that is the real horror of the film, because the monster she hunted for was her own side. The reveal lands harder because of what Michael has just heard in captivity: Viktor killed his own daughter Sonja after she fell in love with Lucian. A ruler who destroys his own child over a forbidden bond has no problem erasing a Death Dealer's family and keeping the secret. Selene's turn on Viktor is the moment the film's two threads, the war and the lie, finally meet.
Lucian's death is the cost. Kraven kills him, which leaves the Lycans without a leader, and it could be argued that the film wants the Lycan cause to feel tragic. Lucian is the one who bit Michael, and Singe had just explained that Michael descends from a common ancestor of both races and could become a hybrid of the two. Lucian's plan dies with him, yet Michael is already the proof that it could work. Selene and Michael escape as outcasts from both the vampire and Lycan worlds, which is why the sequel opens with them on the run.
It is worth comparing how the series treats this history later. Underworld rise of the lycans, directed by Patrick Tatopoulos with Wiseman producing and shot largely in New Zealand according to Screen Daily, plays out the Lucian and Sonja romance as a prequel set centuries earlier. Lucian is born a Lycan but can take human form, and the vampires use the Lycans as enslaved labor. Seen after that film, Viktor's cruelty in the 2003 film reads as the end of a long habit rather than a sudden outburst.
Then comes the final beat. Was Marcus Corvinus the first vampire? Nothing in Underworld (2003) says so. He is introduced as another powerful elder who awakens as a hybrid, and the film leaves it there. Underworld evolution picks up the thread with Marcus waking early and killing Kraven and his men, and its setup names William Corvinus as the first werewolf, held in a secret prison by Viktor. For the order of these events across the series, see the underworld timeline and the corvinus bloodline guide.
