I think you're mistaking hostility for non-sapience. Not every Kaiju out there lives in human civilization, and not every Kaiju wants to live among them as equals. The Leviathans have their own underwater society, and they tend to see humans as little more than pests or prey. I don't know if there are any peaceful Leviathans. If there are, they stick to the depths of the sea. The ones that show up near the surface almost always attack on sight, and rarely attempt to communicate, but signs indicate they could be even more intelligent than humans in some ways.

Pestilence is definitely a weird case, and a tragic one. It was an Elder Kaiju which could only sustain itself by consuming diseased flesh or plant matter. It used to be revered long ago as a sort of guardian against plague, as it would consume the terminally ill to prevent the illness from spreading. A grim role, one I'm told it never enjoyed.

Then, medicine got better. Most of the more serious illnesses it would feed on were cured or prevented. Terminal illnesses stopped being terminal. Since Pestilence requires diseased bodies, it couldn't find enough food to sustain itself. So it started to starve.

What we see now is a hunger-crazed monstrosity, desperately seeking a meal. It can think of nothing else, and the diseases within it's body have putrefied it's flesh and twisted its mind. Rot took it's body, and the fungal parasites it had consumed started growing in it's skull. It's one goal is now to spread disease in an effort to cause a plague. Maybe something remains in its mind, hopeful that by doing this, it will be able to finally sate it's hunger.

All Kaiju as we know them are Sapient. The Elders, the Leviathans, the Legends, the Colossi, and the Xenoforms are all of human intelligence or greater. However, I've heard of possible non-sapient hyperfauna in the world that might be considered Kaiju, but they aren't actually related to the rest of us.