Wanted to put some hints in the game for how to win, but didn't prioritize that. So...
SPOILER:
If you wanted to find the ending, you will need to "save" souls by having them go over the correct small altar in the center (without killing them). There are 5 small areas you can get to depending on how many you save or kill. But if you don't want to do potentially tedious trial and error to find the true ending...
2ND SPOILER:
You need to kill half and save half (4 and 4).