The most likely explanation is that the lab was engaging in risky scientific research, because they wanted to do ground-breaking science and were asking themselves whether they could, not whether they should.
Then they made a mistake, had a gap in the protocols, didn't follow them out of laziness, or some other accident, and infected themselves.
As attractive as such theories sound, pandemics are unpredictable enough that noone sane would start one to further some specific goal. At a large scale, there are rarely true winners in a pandemic (or war).