Insecticide or a Hammer to kill insect vermin??

Insecticide or a Hammer to kill insect vermin??? The disconnect between Precursor technology capability and rationale I do not understand why the Precursors are trying to eliminate humans with Kaiju (a hammer). It is implied (or I have misunderstood) that the Precursors are advanced enough to manufacture an inter-dimensional bridge(the “throatâ€) to earth and biologically engineer the Kaiju. Gottlieb: Here is our universe, and here is theirs. And this is what we call the 'Throat', a passing between the breach and us. We know that it's atomic in nature. I predict that the increased traffic will force the breach to stabilize and remain open long enough to get the device through and collapse its structure. And later Dr. Newton Geiszler: Why do we judge the Kaiju on a category system? It's because each one is completely different from the next, right? You see what I'm saying? One looks like a shark and one looks like a fish... Stacker Pentecost: Dr. Geiszler, let's get to the point. Gottlieb: Yes. Dr. Newton Geiszler: The point is, I don't think they're all completely different after all. Uh...here's some samples I collected, alright? [taking out some of his Kaiju samples] Dr. Newton Geiszler: Now this one here was harvested in Sydney. And this was harvested in Manila, six years ago. They have the same exact DNA, they're clones. If so, why even bother engineering Kaiju? It is suggested in the film that the Kaiju have been sent to soften human defenses for a full scale invasion and that the precursors ultimate goal is the consumption of earth and its resources. Dr. Newton Geiszler: "These beings, these masters, they're colonists. They overtake worlds, they just...they just consume them and then they-they move on to the next. And they've been here before, as sort of a trial run. It was the dinosaurs, but the atmosphere wasn't conducive, right? So they waited it out, and they waited it out. And now, you know, with the ozone, the pollution, the carbon monoxide, polluted waters...well we practically terraformed it for them. Cause now they're coming back, and it's perfect. See the first wave, that was just the hounds, categories 1 through 4, it was nothing! They're sole purpose was to aim for the populated areas and take out the vermin, us! The second wave, that is the exterminators, and they will finish the job. And then, the new tenants will take possession. In my opinion the precursors cannot just nuke all the humans because it might damage the “perfectly terraformed earthâ€. But why wouldn’t they be able to just infect humans with a deadly bio-weapon (Insecticide) like a virus or something that kills humans via corrupting our DNA send it through the throat and upon delivery; the vermin, us just drop like flies. The more I think about it the more it seems rational and less sense the story makes. In addition wouldn’t enough toxic Kaiju Blue sent through the throat & breach into our ocean be enough to kill humans or at least diminish enough of the planets water to eradicate most of the vermin. So that leads me to think the precursors cannot just bio-weapon all the humans because it will damage the “perfectly terrformed earth. There seems to be a disconnect between Precursor technology, capability and rationale. They can bio-engineer Kaiju (a hammer) but not an insecticide to take us out without affecting how perfectly terraformed the planet is, that seems implausible. Is it possible that the Precursors do not know anything about humans? Not likely…. right?? The very engineering of Kaiju doesn’t really seem logical and that’s ultimately the point of this post to express the disconnect between Precursor technology, capability and rationale. Again I find it unreasonable to bio-engineer Kaiju when the precursors could eliminate humans by many other means (even if the counter-argument is the “damage the perfectly terraformed Earth†counter point.