What do you propose then? Ghouls are infinitely stronger than normal humans. From what we´ve seen, the majority of ghouls doesn´t care about human life. So what do we, as humans, do? Well, what do we do with dangerous animals? We put them in cages/special areas or kill them when they get to close to our everyday-life. By that metric, there´s nothing wrong with the investigators´ treatment against ghouls.