There is no mention in the second film that the release of the virus was accidental. The incident in the Hive when Spence broke the vial with the T-virus was intentional; he had multiple reasons to contaminate the facility. Remember that Spence was also a secret operative for the Corporation, and it is likely (although never mentioned) that he was instructed by its authorities to preemptively release the virus, once he discovered that Alice and Lisa planned to infiltrate the Hive and expose its research to the media. It appears that the actions of Umbrella in re-opening the Hive, not taking any security measures, and eliminating civilians are signs of their grander scheme to bring about the armageddon. Aside from these two speculations, the Final Chapter makes it very clear that "he [Dr. Isaacs] released it [the T-virus] deliberately". It's revealed in the scene, when Alice and the team enter the Hive and speak to the Red Queen. Therefore, there should be doubt that the Corp. started the outbreak purposely.