Cockroaches are a basic example, and they will still respond to some basic stimuli. Also, it suddenly has very limited resources and senses, so it has less stimuli to respond to and with, and fewer basic needs it can fulfill. Since basic needs and response to stimuli are the backbone of most cockroach behavior, of course it will be far less active. A more evolved variant of that same model, with a larger body and thus more space to work with, could have either a more redundant anatomy, or a far more sophisticated set of stimuli responses. Functions, such as "chase", "catch", "kill", could be hardwired in many places, as well as the associated biological functions which support those, and you have one very resilient terror agent, even without armor.
Anyways, the aliens are supposed to have some level of distributive intelligence on a societal level, so would it be such a leap to have some semblance of that on a biological level? There would be anatomical limitations, especially among the aliens who are evolutionarily more humanoid, but with sufficient genetic variety there would be some with anatomies more inclined to that structure.