They DID try to change the past.
Ok I've got to start off with a confession, I'm a huge movie fan and I literally only JUST watched this flick for the first time. I thoroughly enjoyed it and even the "age" of the movie didn't detract from the fantastically intrinsic plot. Very solid movie indeed, with some very good, but somewhat overacted, performances.
Now, onto my point....
As soon as I finished the movie I jumped onto the boards to see what peoples opinions were and the ending slightly differs from my own. At the VERY end of the movie.....
The scientist is quoted as saying she is the insurance. To me this says that the future team did indeed try to change the future by killing the guy who kicked it all off. They were fairly sure it wasn't going to work, as they don't believe the timeline can be changed (something we witness throughout the movie) and therefore the scientist also went back in time as an insurance policy to make sure that if the virus outbreak still happens, they will have an original sample to cure the "future" population.
For me, this would be the most rounded ending for the movie. The conscience is clear as they tried to do the right thing, it couldn't/wouldn't happen so the next best solution was put into play.