Because it's stupid? They basically took one of the most badass characters in RE and mad him into a pussy who got beat up by a girl in fucking hand to hand combat. It's not that they were equally strong in the movie, I knew that much already. But why fucking go there? Not only did they make Jill into a useless side character (when the movie's plot and characters all directly relate to her much more than the other chick) but they fucked with what could've been a cool RE movie.
I can see people enjoying it if they haven't played any RE games, cause you don't know wtf to expect and how things should've happened. But like anyone who's a fan of the games (who arguably, are the reason game movies are made) was disappointed greatly.
Game movie fail because they both fail at the "movie" part, as well as the "game" part. RE2 movie was ok on its own, and it did have a RE feel to it.
About the Nemesis scene, I would lie if I said I remember it well, but I don't remember being anoyed by it. A nemessis vs nemessis h2h would be ok? So why not have both nemessis experiment fight each other? I agree it might have been more fun if Nemessis was running after them with rocket launcher for the whole time, but the path they took doesnt destroy completely the movie.
As for Jill being useless, I seriously dont remember any of the character doing much in any of the game (havent played RE4 tho), they are mostly trying to survive and escape the place.