Something I find interesting is that she says that the people who they burned in the ceremony are in a better place and that the fire cleansed them of the sins of the world, following that she says that there is only one hell, the one we live in now.
In which case, why not just burn everyone including yourself and free yourselves from the sins of the earth, spare everyone the wrath of winter/hell and send them off to a better place? If the world is indeed the only hell, the one in which they live in, then why try to preserve it?
The part I underlined isn't in the video, but I guess Meli's higher purpose is not to save mankind, but to win the war between the Lord of Light, and Lord of Darkness. Hence the reason for not just burning everyone and sparing them of the sins of the world.