This depends on a lot of things: how many committed omnicidalists there are; how much financial resources they have (untraceable crypto doesn't help you if you'

This depends on a lot of things: how many committed omnicidalists there are; how much financial resources they have (untraceable crypto doesn't help you if you're working a dead-end job and only have $5K in your bank account); engineered bioweapons need labs and equipment not just an API key. The probability of your scenario doesn't solely depend on the probability of AI being able and willing to cooperate in it, and it may well be that the non-AI factors outweigh the AI ones in the overall risk of it – which would mean adding AI would be increasing the risk of it less than you think.