Units with the ability "Locust" are untargetable and unkillable. I don't think you can select them with Unit Groups either (someone correct me if I'm wrong). Therefore, if you want to keep the behavior, they can't be "killable".
..However, they all die if the caster of the Locust Swarm dies.
I think you can create multiple units that all cast a custom Locust Swarm that only creates 1 "Locust" and figure out a way to work around that. It would be difficult for sure.