What do they gain from killing humans after making out? It cant be an accident since the "victims" survived at the start.
I think that when they were experimenting in the pool, they saw they needed a little time after reproducing. And if kept longer in captivity by SBC, men will be too afraid for it to work anymore. Also, we're talking about superpowered individuals born at full maturity and acting on instinct without any education, violence is expected.
It's very common for babies to start clawing at your face and eyes if you don't watch what they do without realizing.
So that first stage is just toddlers not knowing their strength and breaking their toys, but the real dangers is that this group will then encourage that behavior, unlike our heroines.