I found this on the "Resident Evil Wiki":
After escaping from Raccoon City in 2002, Angela and her new friends remained in hiding for some time from Umbrella and U.S. Government agents secretly under their employ. Eventually, the Global T-virus Outbreak began and Angela, Alice and the others joined up with other survivors into The Strike Team, a mismatched collection of survivors journeying across the United States saving as many people as they could from the growing zombie legions. It was during combat with zombies besieging yet another survivor enclave that Samuel J. Isaacs tested his cybernetic control over Alice, implanted via. a chip in her brain during her time in the Umbrella facility. Isaacs commanded Alice to return to Detroit (one of the few cities protected from zombies and mass upheavel), and bring Angela with her (at Chairman Albert Wesker's behest). Upon their arrival, however, Isaacs explained that Angela was of no use to Umbrella's research in finding a permanent T-virus cure, as the strain of T-virus she had within her was an entirely different strain from the one sweeping across the globe. As such Isaacs ordered Alice to kill Angela, which she did so by shooting her in the head, killing her instantly.