Peoples divided by language and custom could still read one another through how they handled the dead, because each believed a soul travels on after death. That shared belief led all of them to treat corpses with respect and to mourn with rituals. Strangers watched, asked about, and sometimes joined each other's funerals, so death rites became a channel of communication.
When different cultures live side by side, their ways of handling death gradually mix. Practices from very different traditions merge, and beliefs that once contradicted each other are slowly reconciled. Burial and mourning customs leave a visible record of that blending.
When contact between groups is exploitative, understanding a rival's death customs becomes a tool for power. Once you know what another people holds sacred about the dead, you can use it to impress them, win them over, or horrify them. In colonial encounters, missionaries used this knowledge to gain converts and outsiders used it to pose as healers.
Mutilating enemy corpses can be a calculated message rather than blind rage. Fighters chose it because they knew their enemies feared harm to the body after death. The horror worked precisely because both sides shared the belief that the corpse carries the soul and deserves respect.