Software is able to simulate the effect of chemical compounds / molecules on the human body. This can be used to find drugs that do specific things, or stronger versions of existing drugs. For example, you could look for very strong but very short acting sleeping pills that immediately make you fall asleep, but cause zero grogginess the next day. Or you could optimize antibiotics to have a high half life, so you only have to take them once, instead of 3 times a day for a week, which you can easily forget.

Now think about nerve gas. We have discovered lots of different nerve gas agents and know pretty well how much of each type you need to kill a human. Said software can be used to find new versions of nerve gas that kill with even lesser concentrations. You could also optimize for other variables: Nerve gas that remains on surfaces and doesn't decay by itself for example.

Before, computers were used to make less poisonous chemicals.

Now, the people asking computers to do that realized they can ask the computers to make more poisonous chemicals.

They had an AI that looked for safe drugs by minimizing an estimate of lethality, changed it to ‘maximize’ and the computer spewed known nerve gas agents.