[Blood of Tyrants]

Soft armor does no good against bladed weapons and even if she was wearing plates, it has gaps and doesn’t cover your entire body.

[marktwain]

Soft body armor is pretty good at protecting against slashes.

If the bladed weapon has a sharp point and sharp edges, some soft body armor is not much of a protection against a stabbing attack.

Considering the knife used, I suspect the killer managed to avoid the body armor.