[It's not the Enclave remnants he's afraid to say this in front of, it's her. And Deacon. And Riley. And Ferran.]

Research and Development was tasked with modifying the F.E.V., refining Lieutenant Colonel Doctor Charles Curling's old research into creating a toxin that would be fatal to any human who had suffered from radiation exposure.

We weren't told why, but it wasn't difficult to guess. You don't throw resources into a project like that unless you intend to use it.