A couple of things. First, most forms of soap will kill bacteria. Molecules of soap physically break down the cell walls of bacteria, destroying them. For the most part, anti-bacterial soap is just a marketing gimmick. There is no evidence that it is any more effective at killing bacteria than regular soap under real life conditions. Second, insect populations develop resistance to Bt fairly quickly. It's possible you are dealing with a Bt-resistant population of cabbage worms if they've been exposed to it for several years.
