[RedKey]

Spoiler for just in case:

I see now, thanks for the explanation.
This means the whole war was fought just out of fear and resentment — the Flame Hazes feared the legendary Snake of the Festival would be an evil being and undo reality or create something dangerous for the balance of the Power of Existence (or beared their personal grudges against Denizens and wanted to kill them all anyway); the Snake, that once wanted to solve things peacefully, now feared the Flame Hazes would oppose and seal him again if he tried to talk with them so attacked them first. It's all about mutual misunderstandings that led to loss of lives. This is...very sad, indeed.

However, I'd put the greater part of the blame on the ancient Flame Hazes.