How would it be lethal? Please specify. What would that theoretical entity be able to do that hasn't been done many, many times before?
Before we discussed how important security was, we got insurance, we made libraries and products, we used compliance software, etc. Except how honest were we about all that stuff? How much risk was actually in the air, and what was keeping us accountable on security in either direction of over or under-investment?
Now a reckoning is here. The potential to be attacked might actually translate to being attacked.
Design a novel virus which is far more lethal than COVID-19 (Ebola, smallpox, take your pick) and can evade existing vaccines.
I was writing a long reply about how even meat bags have amassed enough money and power to rival elected governments. I don't think it's beyond the realm of possibility to think an AI could do that.
Couple that with mass unemployment in an incredibly vast, diverse population of individuals with individual moral boundaries willing to do whatever for money.
And then.. figured that you must be aware because it's been explored constantly in sci-fi for many, many years.
Let's hope for the culture at least.