it's just death, death, death for its own sake

The key is how the boy learns to confront death (and life) through his father contra his mother.

To his father, it's a "kill or be killed" kind of world. He tells himself that living is about survival, thriving on the violence it necessities and the glory that comes with it. Outside, he confronts death as a means of survival; at home, he avoids it.

The village they live in is built on the same principle. They have their own mementos to death, but these mementos, like the rituals around hunting, exists to ensure them that death, through violence and isolation, can be kept at bey.

When the boy arrives with his dying mother at the skull temple, he has to confront death in a way that his father and the village has ceased to do. In the village, the only graves we see are simple crosses near the gate; they're not so much in memory of the dead, as they're a warning of the danger that lurks outside the village. The temple, on the other hand, is built on remembrance: not of death as an end to life, but of the dead as living human beings.

Memento mori, Memento amoris.

I think it's basically about maintaining some semblance of humanity in a world that, having grown accustomed to death, has embraced barbarism thinking it's the only means of survival.