** SPOILER warning for anyone who has not watched the movie or read the book **
In the novel, at the end, it is revealed that the virus was never meant to kill anyone, but instead made a part of the world's population infertile, which is quite a good ending in my opinion. It provides a solution to the problem of over population, while not killing anyone. There couldn't have been a better ending. Also actually releasing the virus way before the alleged time made a lot of sense and the fact that almost everyone has the virus at the end provides a sort of meaningful conclusion.
In the movie, the virus was in fact going to kill a lot of people and paints Zobrist as a terrorist. Also the ending seems incomplete as the problem still stands at the end, earth is still doomed.
So why would they change such a good ending and replace it with a much poorer one? Did the idea of a part of population infertile seemed a little controversial to the Studio?