Been a while since I last saw the movies, but as far as I recall, the point is not saving the world or anything. They are trying to find shelter, to survive. I can think of two logical reasons two attack Umbrella :
1. Their HQ is the safest place on earth. Taking over it would ensure survival, plus they are likely to find the resources and the equipment to secure a perimeter on the surface and thus live outside with the sunshine and all that sh*t, if they feel the need to live outside. Or they could try to use Umbrella's resources to destroy most/all of the bio-weapons and the infected through a tactical approach. In any case, HQ is a place worth risking your life for.
2. Umbrella turned Alice into a test subject, cloned her like a damn mouse and treated her like one too. The others? How about their entire world got destroyed by Umbrella? Considering the fact, as stated above, that Umbrella also holds the only places they can survive, they have every reason to go and shoot the hell out of that HQ.