My theory is, lunas were filled with rage and Muhan was an easy target. They wanted to do something to get their power back, but in order to get their power, they had to commit a sin, which according to them is not a big deal compared to what happened to them. In conclusion, Systematic oppression builds evil.
Dracula in Castlevania is a good example for systematic oppression making a good man into an evil man. At the end, the church got to wipe their hands off because the bloodshed of a man in pain was bigger than the sins they caused.

Imagine ur family is into human trafficking and exploitation and the cult opposing them punishes u and tortures u to death