No, your premise is wrong. You're implying that monsters are the same as humans, which is not true. Before Rimuru appeared, goblins were wild savages. Orcs destroyed whole landscapes eating everything. Even the ogres (or whatever Hina and co are) used to be more mindless, aggressive monsters.
Humans in this world only knew monsters as dangerous, wild beasts and that justifies killing monsters. Racism in our real world was a big issue DESPITE white and black people being equally intelligent. In this world, humans were intelligent, monsters were not, or when they had a bit of intelligence, they were still overtly aggressive.
Now you're telling me that humans should just watch as a gathering of monsters grows stronger and stronger? Fuck that shit. Their attack was ultimately justified, only the way they went about it in the beginning wasnt, BUT: That was a plan that only a small group knew about. The vast majority of them, basically soldiers, still believed, and for good reason, that they were up against wild beasts.
Taking chances and hope that the monsters leader always remains a "good" monster? I cerntainly wouldn't have taken that chance without the knowledge I have as an outside-viewer.